[Phase 7F] Framebuffer diff rendering: flat array + flush diffs only

[fissible]

What

Replace per-render /dev/tty writes with a double-buffered framebuffer. Only changed cells are emitted per frame.

Architecture:

• Two flat 1D arrays: _SF_FRAME_CURR[row*COLS+col] and _SF_FRAME_PREV[row*COLS+col] — each cell stores the rendered string for that position (character + ANSI codes).
• All render functions write to _SF_FRAME_CURR instead of /dev/tty.
• shellframe_screen_flush diffs CURR vs PREV, emits only changed cells as positioned escape sequences (\033[row;colH), then copies CURR → PREV.
• shellframe_screen_clear resets both buffers and clears the terminal.

Migration note: Check src/screen.sh for any scaffolding already in place from task B.

Recommended approach — migrate incrementally:

1. Add framebuffer array init + shellframe_screen_flush (diff + emit)
2. Migrate one render function (e.g. shellframe_panel_render) to write to buffer; run tests
3. Migrate remaining render functions one at a time, running tests after each
4. Remove all direct /dev/tty writes from render paths
5. Call shellframe_screen_flush at end of each app loop iteration in shellframe_shell

Files

• src/screen.sh — framebuffer arrays, shellframe_screen_flush, clear reset
• All widget render functions — write to _SF_FRAME_CURR instead of /dev/tty
• src/shell.sh — call shellframe_screen_flush at end of loop

Done criteria

• All render output goes through shellframe_screen_flush
• Only changed cells emitted per flush — verified by a test that a no-change flush produces zero output
• No direct /dev/tty writes remain in render functions
• All existing assertions still pass after full migration
• Tests: no-change flush emits nothing; single-cell change emits only that cell; full clear resets both buffers

IO/timing note (CLAUDE.md §5)

The no-change flush test must exercise an actual flush call — not a mock. Terminal behavior (what triggers output) must be verified in a real environment.

Effort

XL (2–3 days)

Deps

• [Phase 7B] Dirty-region rendering: widget dirty flags + conditional re-render #30 (Phase 7B — dirty-region rendering must be in place first)
• This is the largest Phase 7 task; do not start until [Phase 7B] Dirty-region rendering: widget dirty flags + conditional re-render #30 is verified complete