refactor: extract window-geometry helpers shared by F9 and F12

`ai-vterm.el` (F9) and `eshell-vterm-config.el` (F12) both grew the same geometry-preservation pattern: classify a window's position, capture its body size, map cardinal direction to its frame-edge variant. The shared helpers were sitting as near-duplicates in both modules. With two real consumers established, the abstraction has the right shape. I pulled them into `cj-window-geometry.el`.

The new module exposes three pure helpers:

- `cj/window-direction` returns right/below/left/above based on edges relative to `frame-root-window`. Takes an optional DEFAULT for the single-window-frame fallback so each consumer picks its own (ai-vterm wants 'right, vterm-toggle wants 'below).
- `cj/window-body-size` returns body-cols (right/left) or body-lines (below/above). Same body-vs-total reasoning as before: divider-independent, matches what the user sees.
- `cj/cardinal-to-edge-direction` maps right/left/below/above to rightmost/leftmost/bottom/top, used by each consumer's `display-saved` action.

`ai-vterm.el` and `eshell-vterm-config.el` now `(require 'cj-window-geometry)` and call the shared helpers directly. The consumer-specific `capture-state` and `display-saved` bodies stay in each module because they bind to consumer-specific state vars. Extracting those would either need parameter-passing-via-symbol or a macro, both heavier than the duplication they would remove.

Tests: 15 in `test-cj-window-geometry.el` covering all four directions, body-size on both axes, cardinal-to-edge mapping, default-arg fallback, and the unknown-direction nil case. Deleted `test-ai-vterm--window-geometry.el` (now redundant) and trimmed four duplicate window-direction/size tests from `test-vterm-toggle--display.el`. Net LOC: each consumer ~40-50 lines lighter, with the new module + tests paying roughly half that back. Full make test green. make validate-modules green.

3 files changed, 98 insertions, 90 deletions

diff --git a/modules/ai-vterm.el b/modules/ai-vterm.el
index 9b47e33d..e817f441 100644
--- a/modules/ai-vterm.el
+++ b/modules/ai-vterm.el
@@ -36,6 +36,7 @@
 
 (require 'cl-lib)
 (require 'seq)
+(require 'cj-window-geometry)
 
 (declare-function vterm "vterm" (&optional buffer-name))
 (declare-function vterm-send-string "vterm" (string &optional paste-p))
@@ -228,57 +229,19 @@ and a fraction-of-frame produces the wrong size on replay
 (squeezes the other windows).  An integer is unambiguous, at the
 cost of not auto-scaling if the frame itself resizes.")
 
-(defun cj/--ai-vterm-window-direction (window)
-  "Return the side WINDOW occupies in its frame.
-
-Returns one of right, below, left, above.  Falls back to right when
-WINDOW fills its frame's root area (single-window or atypical
-layout), since right is the module's default split direction.
-
-Comparison uses `frame-root-window' edges rather than frame edges so
-the minibuffer doesn't make every full-area window look like it
-fails to span the full height."
-  (let* ((root (frame-root-window (window-frame window)))
-         (edges (window-edges window))
-         (root-edges (window-edges root))
-         (left (nth 0 edges))
-         (top (nth 1 edges))
-         (right (nth 2 edges))
-         (bottom (nth 3 edges))
-         (root-left (nth 0 root-edges))
-         (root-top (nth 1 root-edges))
-         (root-right (nth 2 root-edges))
-         (root-bottom (nth 3 root-edges))
-         (spans-full-width (and (= left root-left) (= right root-right)))
-         (spans-full-height (and (= top root-top) (= bottom root-bottom))))
-    (cond
-     ((not spans-full-width) (if (= left root-left) 'left 'right))
-     ((not spans-full-height) (if (= top root-top) 'above 'below))
-     (t 'right))))
-
-(defun cj/--ai-vterm-window-size (window direction)
-  "Return WINDOW's body size in cols (right/left) or lines (below/above).
-
-Returns body width or body height -- the count of characters
-visible in the text content area, independent of fringes,
-scrollbars, or window dividers.  See `cj/--ai-vterm-last-size' for
-why body size, not total size, is the right thing to capture."
-  (if (memq direction '(right left))
-      (window-body-width window)
-    (window-body-height window)))
-
 (defun cj/--ai-vterm-capture-state (window)
   "Capture WINDOW's direction and size into module-level state.
 
 Sets `cj/--ai-vterm-last-direction' and `cj/--ai-vterm-last-size'
 so a subsequent F9 display can restore the user's chosen orientation
-and size.  Called at toggle-off (just before `quit-window' tears the
-window down).
+and size.  Called at toggle-off (just before the window is torn
+down).  The default direction is 'right -- the module's side-panel
+default.
 
 Does nothing when WINDOW is not live."
   (when (window-live-p window)
-    (let* ((dir (cj/--ai-vterm-window-direction window))
-           (size (cj/--ai-vterm-window-size window dir)))
+    (let* ((dir (cj/window-direction window 'right))
+           (size (cj/window-body-size window dir)))
       (setq cj/--ai-vterm-last-direction dir
             cj/--ai-vterm-last-size size))))
 
@@ -333,12 +296,8 @@ stripped so the saved-state values control placement -- callers
 shouldn't specify direction or size in the rule when this action is
 used."
   (let* ((direction (or cj/--ai-vterm-last-direction 'right))
-         (edge-direction (pcase direction
-                           ('right 'rightmost)
-                           ('left 'leftmost)
-                           ('below 'bottom)
-                           ('above 'top)
-                           (_ 'rightmost)))
+         (edge-direction (or (cj/cardinal-to-edge-direction direction)
+                             'rightmost))
          (size (or cj/--ai-vterm-last-size cj/ai-vterm-window-width))
          (size-key (if (memq direction '(right left))
                        'window-width
diff --git a/modules/cj-window-geometry.el b/modules/cj-window-geometry.el
new file mode 100644
index 00000000..88fa83d4
--- /dev/null
+++ b/modules/cj-window-geometry.el
@@ -0,0 +1,81 @@
+;;; cj-window-geometry.el --- Pure window-geometry helpers -*- lexical-binding: t; -*-
+
+;; Author: Craig Jennings <c@cjennings.net>
+
+;;; Commentary:
+
+;; Pure helpers for classifying a window's position in its frame and
+;; computing body sizes.  Shared between `ai-vterm.el' (F9 dispatch)
+;; and `eshell-vterm-config.el' (F12 dispatch); the geometry-
+;; preservation pattern in both modules captures direction + body
+;; size at toggle-off and replays them on the next toggle-on.
+;;
+;; All functions are pure: they read window/frame edges and return
+;; classifications.  No side effects, no state.  Consumers wrap them
+;; with consumer-specific state vars and display logic.
+
+;;; Code:
+
+(defun cj/window-direction (window &optional default)
+  "Return the side WINDOW occupies in its frame.
+
+Returns one of right, below, left, above.  Falls back to DEFAULT
+(or right when DEFAULT is nil) when WINDOW fills its frame's
+root area.  Comparison uses `frame-root-window' edges so the
+minibuffer doesn't make every full-area window look like it
+fails to span the full height."
+  (let* ((root (frame-root-window (window-frame window)))
+         (edges (window-edges window))
+         (root-edges (window-edges root))
+         (left (nth 0 edges))
+         (top (nth 1 edges))
+         (right (nth 2 edges))
+         (bottom (nth 3 edges))
+         (root-left (nth 0 root-edges))
+         (root-top (nth 1 root-edges))
+         (root-right (nth 2 root-edges))
+         (root-bottom (nth 3 root-edges))
+         (spans-full-width (and (= left root-left) (= right root-right)))
+         (spans-full-height (and (= top root-top) (= bottom root-bottom))))
+    (cond
+     ((not spans-full-width) (if (= left root-left) 'left 'right))
+     ((not spans-full-height) (if (= top root-top) 'above 'below))
+     (t (or default 'right)))))
+
+(defun cj/window-body-size (window direction)
+  "Return WINDOW's body size on the axis matching DIRECTION.
+
+Returns body-width (columns) when DIRECTION is right or left.
+Returns body-height (lines) when DIRECTION is below or above.
+
+Body size, not total size, is the right thing to capture for
+geometry replay: total-width includes the right-side divider when
+the window has a right sibling but excludes it at the frame edge,
+so a captured rightmost window replayed into a middle position
+would leave the body 1 col short.  Body size is divider-
+independent and matches what the user actually sees."
+  (if (memq direction '(right left))
+      (window-body-width window)
+    (window-body-height window)))
+
+(defun cj/cardinal-to-edge-direction (direction)
+  "Map cardinal DIRECTION to its `display-buffer-in-direction' edge variant.
+
+Returns rightmost/leftmost/bottom/top for right/left/below/above
+respectively.  Returns nil for any other input.
+
+The edge variants route splits relative to the frame's main
+window rather than the selected window, so a toggle-on lands at
+the same frame edge regardless of which window is selected.  The
+cardinal variants would split the selected window's tree branch
+instead, putting the new window mid-frame in multi-window
+layouts."
+  (pcase direction
+    ('right 'rightmost)
+    ('left 'leftmost)
+    ('below 'bottom)
+    ('above 'top)
+    (_ nil)))
+
+(provide 'cj-window-geometry)
+;;; cj-window-geometry.el ends here
diff --git a/modules/eshell-vterm-config.el b/modules/eshell-vterm-config.el
index ed257893..34738fa1 100644
--- a/modules/eshell-vterm-config.el
+++ b/modules/eshell-vterm-config.el
@@ -261,6 +261,7 @@ ai-vterm.el is loaded."
 
 (require 'cl-lib)
 (require 'seq)
+(require 'cj-window-geometry)
 
 (defcustom cj/vterm-toggle-window-height 0.7
   "Default fraction of frame height for the F12 vterm window.
@@ -306,43 +307,14 @@ FRAME defaults to the selected frame.  Minibuffer is excluded."
               (cj/--vterm-toggle-buffer-p (window-buffer w)))
             (window-list (or frame (selected-frame)) 'never)))
 
-(defun cj/--vterm-toggle-window-direction (window)
-  "Return the side WINDOW occupies in its frame.
-
-Returns one of right, below, left, above.  Falls back to 'below
-(F12's traditional bottom split) when WINDOW fills its frame's
-root area.  Comparison uses `frame-root-window' edges so the
-minibuffer doesn't make every full-area window look like it
-fails to span the full height."
-  (let* ((root (frame-root-window (window-frame window)))
-         (edges (window-edges window))
-         (root-edges (window-edges root))
-         (left (nth 0 edges))
-         (top (nth 1 edges))
-         (right (nth 2 edges))
-         (bottom (nth 3 edges))
-         (root-left (nth 0 root-edges))
-         (root-top (nth 1 root-edges))
-         (root-right (nth 2 root-edges))
-         (root-bottom (nth 3 root-edges))
-         (spans-full-width (and (= left root-left) (= right root-right)))
-         (spans-full-height (and (= top root-top) (= bottom root-bottom))))
-    (cond
-     ((not spans-full-width) (if (= left root-left) 'left 'right))
-     ((not spans-full-height) (if (= top root-top) 'above 'below))
-     (t 'below))))
-
-(defun cj/--vterm-toggle-window-size (window direction)
-  "Return WINDOW's body size in cols (right/left) or lines (below/above)."
-  (if (memq direction '(right left))
-      (window-body-width window)
-    (window-body-height window)))
-
 (defun cj/--vterm-toggle-capture-state (window)
-  "Capture WINDOW's direction + body size into module-level state."
+  "Capture WINDOW's direction + body size into module-level state.
+
+Default direction is 'below to match F12's traditional bottom
+split when WINDOW fills the frame's root area."
   (when (window-live-p window)
-    (let* ((dir (cj/--vterm-toggle-window-direction window))
-           (size (cj/--vterm-toggle-window-size window dir)))
+    (let* ((dir (cj/window-direction window 'below))
+           (size (cj/window-body-size window dir)))
       (setq cj/--vterm-toggle-last-direction dir
             cj/--vterm-toggle-last-size size))))
 
@@ -359,12 +331,8 @@ in a `(body-columns . N)' / `(body-lines . N)' cons so the body
 width or height is set explicitly, divider-independent.  A float
 size passes through as a fraction of the new window's parent."
   (let* ((direction (or cj/--vterm-toggle-last-direction 'below))
-         (edge-direction (pcase direction
-                           ('right 'rightmost)
-                           ('left 'leftmost)
-                           ('below 'bottom)
-                           ('above 'top)
-                           (_ 'bottom)))
+         (edge-direction (or (cj/cardinal-to-edge-direction direction)
+                             'bottom))
          (size (or cj/--vterm-toggle-last-size cj/vterm-toggle-window-height))
          (size-key (if (memq direction '(right left))
                        'window-width