fix(animation): animate both paws concurrently (#78)

[Aestylis]

Fixes #78.

Problem

Only one paw animates at a time. Pressing a left-side and a right-side key together moves just one paw (the last pressed) — the both-down frame is never shown.

Root cause

The input child overwrites a single shared last_key_code, and the animation thread maps that one code to frame 1 or 2 with a single hold_until. The existing bongo-both-down.svg frame is unreachable.

Approach

The issue suggested separate surfaces per paw. Since the artwork is whole-cat frames (including an existing both-down frame) rather than isolated paw layers, I took a lighter route that needs no new surfaces or art:

• Lock-free single-producer/multi-consumer ring buffer carries every keycode from the input child to each monitor's animation thread.
• Two independent per-paw deadlines; the displayed frame is derived from which paws are live (both → both-down).
• Retires last_key_code/any_key_pressed, which also fixes a multi-monitor reset race on the old shared flag.

Testing

• New unit tests: ring (wraparound, multi-consumer, overflow-drop) + frame mapping.
• make test, make format-check, make release all clean.
• ThreadSanitizer: no races on the ring under two-hand stress typing.
• Valgrind: 0 definitely/indirectly lost.
• Verified live on KWin/KDE (Wayland): both paws animate, both-down confirmed.

Net: +337/−141 across 11 files; no new dependencies; ~260 bytes shared memory (removes two prior mmaps).