[Phase 7B] Dirty-region rendering: widget dirty flags + conditional re-render

[fissible]

What

First: verify whether this is already complete. v0.1.0 shipped "dirty-region rendering — skip screen_clear on cursor movement + button toggle". Check whether the full spec below is implemented before writing any new code. Close this issue if it is.

Full spec:

• Each widget exposes a dirty flag (or the app loop tracks it centrally) so shellframe_screen_clear is only called when a full repaint is needed — not on every key event.
• Render functions continue writing directly to /dev/tty — no API break. Framebuffer migration is task F.
• The shellframe_shell main loop checks dirty state before triggering re-render; a no-op key event that changes no state should not cause a screen repaint.

Files

• src/screen.sh — dirty flag helpers if not already present
• src/shell.sh — conditional re-render in main loop
• Widget render functions — mark dirty on meaningful state changes (selection move, scroll, text edit)

Done criteria

• shellframe_screen_clear is NOT called on every key event — only when a full repaint is needed
• Widget dirty state is set on meaningful state changes
• App loop skips re-render when nothing is dirty
• No existing widget API changes — callers unaffected
• Regression tests covering the dirty → render → clean cycle
• All existing assertions still pass

IO/timing note (CLAUDE.md §5)

"What sets dirty" and "what triggers redraws" are terminal behavior claims. Write a test that confirms a no-op key event does not invoke shellframe_screen_clear — do not rely on code inspection alone.

Effort

M (~half day) — or XS if already done in v0.1.0

Deps

• [P4] Generic app shell #18 (app shell) — closed
• Unblocks Phase 7F (framebuffer diff)

[fissible]

Implemented in commit 6d7c546 on feature/phase-7b-dirty-region. All 1075 unit tests pass (27 files). Docker matrix is blocked by a file-sharing configuration issue on this host (pre-existing, not a regression).

Changes:

• src/shell.sh: Added _SHELLFRAME_SHELL_DIRTY flag, shellframe_shell_mark_dirty(), and _shellframe_shell_draw_if_dirty(); event loop now skips draw when clean
• All 7 V2 widgets call shellframe_shell_mark_dirty on recognized keys
• Dirty integration tests added to all 7 widget test files + test-shell.sh