[Phase 7C] Mouse: SGR sequence parsing + output vars

[fissible]

What

Add mouse enable/disable and SGR sequence parsing to shellframe.

1. shellframe_mouse_enter / shellframe_mouse_exit in src/screen.sh — send \033[?1000h\033[?1006h (enable SGR mouse) on screen enter and \033[?1000l\033[?1006l on screen exit, alongside existing raw mode setup.

2. SGR mouse sequence parsing in shellframe_read_key (src/input.sh) — SGR format: \033[<Pb;Px;PyM (press) / \033[<Pb;Px;Pym (release). Parse into output variables:

   • SHELLFRAME_MOUSE_COL — column (1-based)
   • SHELLFRAME_MOUSE_ROW — row (1-based)
   • SHELLFRAME_MOUSE_BUTTON — button number (0=left, 1=middle, 2=right, 64=scroll-up, 65=scroll-down)
   • SHELLFRAME_MOUSE_ACTION — press or release
   • Set SHELLFRAME_KEY to sentinel constant SHELLFRAME_KEY_MOUSE

Files

• src/screen.sh — shellframe_mouse_enter, shellframe_mouse_exit
• src/input.sh — SGR parsing branch in shellframe_read_key

Done criteria

• Mouse reporting enabled/disabled cleanly around the TUI session
• Left click, right click, scroll-up, scroll-down all parsed correctly into output vars
• SHELLFRAME_KEY_MOUSE sentinel set so callers can branch on mouse vs keyboard
• Unit tests covering SGR parse for press/release/scroll variants
• IO validation required (CLAUDE.md §5): Write an integration test (or manual test with documented result) that sends a real SGR sequence through a PTY and confirms parsed values — string manipulation tests alone are not sufficient

Effort

S (1–2h)

Deps

• [Phase 7A] Input hardening: CSI drain, F1–F12, modifier+arrow sequences #29 (Phase 7A — CSI drain must be in place before mouse sequences are handled)
• Unblocks [Phase 7C] Mouse: SGR sequence parsing + output vars #32 (Phase 7E — mouse routing)

[fissible]

Implemented in commit 9b873b4, merged to main.

src/screen.sh: shellframe_mouse_enter (ESC[?1000h + ESC[?1006h) and shellframe_mouse_exit (reverse sequences).

src/input.sh: SHELLFRAME_KEY_MOUSE sentinel, SHELLFRAME_MOUSE_{BUTTON,COL,ROW,ACTION} globals, SGR parse branch after CSI drain — handles press/release, all button codes (0/1/2/64/65), multi-digit coordinates.

IO validation: 8 subprocess pipe tests in test-input.sh invoke shellframe_read_key with real SGR byte sequences piped through bash stdin, confirming all output vars. 1166/1166 total unit assertions pass.