[Phase 7E] Mouse routing: click-to-focus, click-to-select, scroll-wheel

[fissible]

What

Wire mouse events from parsed SGR sequences (#32) through the hit-test registry (#31) to widget on_mouse handlers.

App shell changes (src/shell.sh):

• When shellframe_read_key returns SHELLFRAME_KEY_MOUSE, call shellframe_widget_at $SHELLFRAME_MOUSE_ROW $SHELLFRAME_MOUSE_COL to identify the target widget.
• If the target widget is not focused, call shellframe_shell_focus_set to move focus to it (click-to-focus).
• Dispatch to the target widget's on_mouse handler: ${PREFIX}_on_mouse "$SHELLFRAME_MOUSE_BUTTON" "$SHELLFRAME_MOUSE_ACTION" "$SHELLFRAME_MOUSE_ROW" "$SHELLFRAME_MOUSE_COL".

Widget on_mouse handlers:

• shellframe_list_on_mouse — click on a row sets selection to that row; scroll-wheel scrolls the list
• shellframe_scroll_on_mouse — scroll-wheel scrolls the container (used by editor, grid)
• Other widgets: click-to-focus only (no additional action needed for v1)

Files

• src/shell.sh — mouse dispatch in main loop
• src/widgets/list.sh — shellframe_list_on_mouse
• src/scroll.sh — scroll-wheel handler (used by list, editor, grid)

Done criteria

• Clicking a widget that is not focused moves focus to it
• Clicking a row in a list selects that row
• Scroll-wheel up/down scrolls a focused list or scroll container
• Clicking outside all registered widgets is a no-op
• Integration test: click on a list item → selection changes to the clicked row
• IO validation required (CLAUDE.md §5): Mouse dispatch must be tested through a real PTY with a synthesized SGR mouse event — not just unit-tested with a mock key value

Effort

M (~half day)

Deps

• [Phase 7C] Mouse: SGR sequence parsing + output vars #32 (Phase 7C — mouse SGR parsing)
• [Phase 7D] Widget hit-test registry: src/hitbox.sh #31 (Phase 7D — hit-test registry)
• Both must be complete before starting this task

[fissible]

Implemented in commit 486e30c.

src/selection.sh: shellframe_sel_set — direct cursor set with clamping.
src/shell.sh: hitbox auto-sync in shellframe_shell_region; SGR parse in _shellframe_shell_read_key; mouse dispatch branch in event loop; shellframe_mouse_enter/exit around TUI session.
src/widgets/list.sh: shellframe_list_on_mouse — click-to-select + scroll-wheel.
src/scroll.sh: shellframe_scroll_on_mouse — generic scroll-wheel for editor/grid.

7 PTY IO-validation integration tests. 1253/1253 total assertions pass.