feat(ai-vterm): F9 toggle/redisplay/pick + persistent split geometry

F9 was a single command that always opened the project picker. Three small frustrations stacked up. With one claude buffer open and not visible, F9 was a redundant prompt to pick a project that already had a session. With claude visible, there was no way to bury it without M-x quit-window. With two projects' buffers alive, swapping between them was a buffer-switch chore.

F9 is now a dispatch:

- Claude visible in this frame: quit the window (toggle off) and capture the geometry first.
- Exactly one claude buffer alive but hidden: re-display it (DWIM single-buffer case).
- Zero or two-plus alive: fall through to the project picker.

C-F9 is the always-pick-project entry point for explicit project switches. M-F9 is a buffer picker over the alive claude buffers. If a claude window is currently shown, the picked buffer replaces it in that window so the split orientation and size carry over. The shown buffer sorts last in the picker with a [shown] marker so RET picks "the other one."

Split geometry persists across toggles. Two module-level vars (cj/--ai-vterm-last-direction, cj/--ai-vterm-last-size) capture at toggle-off and feed a custom display action. After M-S-t flips claude from right to bottom, F9 toggle-off-then-on returns it at the bottom. After a mouse resize, the next toggle restores that fraction. State is per-session. Restarts reset to default right/0.5.

Two display-buffer fixes came out of testing:

- save-window-excursion around (vterm name) keeps the dashboard from being buried on a fresh F9 at startup. vterm calls pop-to-buffer-same-window internally, which would otherwise replace the selected window's buffer before the alist could route the new one.
- The action chain swaps display-buffer-use-some-window for a more specific cj/--ai-vterm-reuse-existing-claude. The generic version stole non-claude windows on C-F9 when the user was focused inside claude (claude on bottom, code on top -> new project landed in the code window). The specific version only reuses windows that already show a claude buffer.

I reclaimed C-F9 from the gptel toggle in ai-config.el. C-; a t still binds gptel.

I added eight new test files (claude-buffers, displayed-claude-window, dispatch, pick-buffer-candidates, window-geometry, capture-state, display-saved, reuse-existing-claude) plus a regression test on cj/--ai-vterm-show-or-create for the dashboard-preservation fix. All 73 ai-vterm tests pass and the full make test suite is green.

1 files changed, 84 insertions, 0 deletions

diff --git a/tests/test-ai-vterm--pick-buffer-candidates.el b/tests/test-ai-vterm--pick-buffer-candidates.el
new file mode 100644
index 00000000..99ef7325
--- /dev/null
+++ b/tests/test-ai-vterm--pick-buffer-candidates.el
@@ -0,0 +1,84 @@
+;;; test-ai-vterm--pick-buffer-candidates.el --- Tests for the M-F9 candidate builder -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; The candidate builder is a pure function: given an MRU list of
+;; alive AI-vterm buffers and the currently-displayed buffer (or
+;; nil), it returns an alist of (DISPLAY-NAME . BUFFER) cells.
+;;
+;; Sort rule: non-shown buffers come first in their input order,
+;; then the shown buffer (if it's in the list) appears last with a
+;; \" [shown]\" suffix.  The intent is that the default `completing-
+;; read' selection lands on a non-shown candidate so RET means
+;; \"give me the other one\".
+
+;;; Code:
+
+(require 'ert)
+
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'ai-vterm)
+
+(defun test-ai-vterm--pbc-cleanup ()
+  "Kill any leftover claude-prefixed buffers."
+  (dolist (b (buffer-list))
+    (when (string-prefix-p "claude [" (buffer-name b))
+      (kill-buffer b))))
+
+(ert-deftest test-ai-vterm--pick-buffer-candidates-empty-buffers ()
+  "Boundary: empty buffer list -> empty alist regardless of shown."
+  (test-ai-vterm--pbc-cleanup)
+  (should (null (cj/--ai-vterm-pick-buffer-candidates nil nil)))
+  (should (null (cj/--ai-vterm-pick-buffer-candidates nil 'sentinel))))
+
+(ert-deftest test-ai-vterm--pick-buffer-candidates-shown-nil ()
+  "Normal: shown is nil -> straight alist in input order, no marker."
+  (test-ai-vterm--pbc-cleanup)
+  (let ((b1 (get-buffer-create "claude [a]"))
+        (b2 (get-buffer-create "claude [b]")))
+    (unwind-protect
+        (let ((result (cj/--ai-vterm-pick-buffer-candidates (list b1 b2) nil)))
+          (should (equal result `(("claude [a]" . ,b1)
+                                   ("claude [b]" . ,b2)))))
+      (kill-buffer b1)
+      (kill-buffer b2))))
+
+(ert-deftest test-ai-vterm--pick-buffer-candidates-shown-promotes-non-shown ()
+  "Normal: shown buffer sorts last with [shown] suffix; others first."
+  (test-ai-vterm--pbc-cleanup)
+  (let ((b1 (get-buffer-create "claude [a]"))
+        (b2 (get-buffer-create "claude [b]"))
+        (b3 (get-buffer-create "claude [c]")))
+    (unwind-protect
+        (let ((result (cj/--ai-vterm-pick-buffer-candidates
+                       (list b1 b2 b3) b1)))
+          (should (equal result
+                         `(("claude [b]" . ,b2)
+                           ("claude [c]" . ,b3)
+                           ("claude [a] [shown]" . ,b1)))))
+      (kill-buffer b1)
+      (kill-buffer b2)
+      (kill-buffer b3))))
+
+(ert-deftest test-ai-vterm--pick-buffer-candidates-shown-only-buffer ()
+  "Boundary: shown is the only entry -> single cell with [shown] marker."
+  (test-ai-vterm--pbc-cleanup)
+  (let ((b1 (get-buffer-create "claude [a]")))
+    (unwind-protect
+        (let ((result (cj/--ai-vterm-pick-buffer-candidates (list b1) b1)))
+          (should (equal result `(("claude [a] [shown]" . ,b1)))))
+      (kill-buffer b1))))
+
+(ert-deftest test-ai-vterm--pick-buffer-candidates-shown-not-in-buffers ()
+  "Boundary: stale shown buffer not in list -> all cells are non-shown."
+  (test-ai-vterm--pbc-cleanup)
+  (let ((b1 (get-buffer-create "claude [a]"))
+        (b-stale (get-buffer-create "claude [stale]")))
+    (unwind-protect
+        (let ((result (cj/--ai-vterm-pick-buffer-candidates
+                       (list b1) b-stale)))
+          (should (equal result `(("claude [a]" . ,b1)))))
+      (kill-buffer b1)
+      (kill-buffer b-stale))))
+
+(provide 'test-ai-vterm--pick-buffer-candidates)
+;;; test-ai-vterm--pick-buffer-candidates.el ends here