[Phase 7D] Widget hit-test registry: src/hitbox.sh

[fissible]

What

Build src/hitbox.sh — a widget bounding-box registry for mouse routing.

API:

• shellframe_widget_register name top left width height — registers a widget's bounding box by name. Coordinates are 1-based terminal rows/columns.
• shellframe_widget_at row col — returns the name of the widget whose bounding box contains (row, col), or empty string if none. Last-registered wins on overlap.
• shellframe_widget_clear — clears all registered widgets (call on full repaint).

Implemented as parallel arrays keyed by registration order; name lookup iterates from last to first so later registrations shadow earlier ones.

Files

• src/hitbox.sh — new module (register, at, clear, parallel arrays)
• src/shellframe.sh — source hitbox.sh

Done criteria

• shellframe_widget_register stores bounding box correctly
• shellframe_widget_at returns the correct widget name for a point inside a registered box
• shellframe_widget_at returns empty string for a point outside all boxes
• Overlap behavior: last-registered wins — documented and tested
• shellframe_widget_clear fully resets state
• Unit tests in tests/unit/test-hitbox.sh: single widget, multiple non-overlapping widgets, point outside all, overlap
• All existing assertions still pass

Effort

M (~half day)

Deps

• [P3] Box/Panel primitive #9 (panel) — closed
• [P4] Generic app shell #18 (app shell) — closed
• No dependency on 7A or 7C — can be worked in parallel with the mouse parse chain
• Unblocks Phase 7E (mouse routing)

[fissible]

Implemented in commit e3ea0d4. New src/hitbox.sh with shellframe_widget_register, shellframe_widget_at (last-registered-wins, supports out_var), and shellframe_widget_clear (all or by name). 15 unit assertions. 1151/1151 total unit assertions pass.