feat: implement Backspace-Replay pattern for perfect engine sync
- Add Engine::replay_keystrokes() — creates fresh engine and replays
all keystrokes to compute correct screen output from scratch
- Add Daemon::replay_and_inject() — tracks keystroke history and
screen output, computes diff (backspaces + new text) on each keypress
- Add Daemon::replay_backspace() — pops from history, replays, diffs
- Handle flush chars (space, period, etc.) separately — commit word,
type char, clear history
- Add FocusIn/FocusOut detection for engine reset on focus loss
- Add CPU pinning (P-cores 0-3) + nice(-10) priority boost
- Clean up 9 dead code warnings (unused fields, constants, types)
- Add replay_keystrokes tests for Telex, VNI, and backspace
- 255 tests pass (was 252)
3858aa955c