udp client: register arena:snap + squash:move geckos listeners
Server-emitted arena:snap packets over UDP were being dropped silently
because lib/udp.mjs only listened for tv, fairy:point, and 1v1:move.
The server's "prefer UDP, fall back to WS" path sees the emit succeed
and never falls back, so arena clients never received snapshots — other
players appeared frozen / absent.
Same latent bug for squash:move; adding both for consistency.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>