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.

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