Add spinning donut screensaver with matrix rain

Terminal screensaver written in C, triggered by tmux lock-after-time
(180s idle), bind S, or `donut` alias. Features:

- Spinning torus with precomputed trig tables, randomized rotation
- Matrix rain overlay — donut luminance colors the rain characters
- 64-shade teal foreground palette with gradual dimming over time
- Chunky 7x5 clock with shadow, blinking colon, periodic drift
- Activity time display below clock (read from heartbeat state file)
- Rim lighting and ambient occlusion on donut edges
- RLE escape state machine — only emits SGR on shade transitions
- Row-skip rendering — empty/leading/trailing regions use cursor jumps
- Projection runs at adaptive rate (24fps bright, 6fps dim)
- select()-based frame pacing, deep sleep after 2hr (1fps)
- Resume detection — flushes stale PTY output after sleep/lock
- Mouse click to quit (SGR + X11 protocols, release filtered)
- write_all() retries partial writes, SIGPIPE handled gracefully
- Alternate screen buffer preserves shell scrollback

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

home/.aliases

@@ -40,6 +40,7 @@ alias gr='git r'
 alias gw='git show'
 alias gf='git f'
 alias gp='git p'
+alias donut='tmux lock-session'
 gdm() { local base="${1:-$(git symbolic-ref refs/remotes/origin/HEAD 2>/dev/null | sed 's@refs/remotes/origin/@@' || echo main)}"; git diff "${base}...HEAD"; }
 
 # fzf workflows