Harden multiplayer damage and visibility

[agustin-lowpoly]

Summary

• harden multiplayer remote player presentation, damage feedback, pickups, world intents, and authoritative damage handling
• add deep browser checks for A/B damage, server-authoritative weapon selection, blood particles, pain/death animation, reconnect, and remote visibility continuity
• expand multiplayer protocol/item/world/presentation test coverage

Validation

• pnpm test
• pnpm build
• node test/browser/runMultiplayerDeepChecks.mjs --map e1m7 --timeout-ms 90000 --json-out bench/results/quake/multiplayer-deep-checks-e1m7-final-predeploy.json
• node test/browser/runMultiplayerBrowserSmoke.mjs --map e1m7 --clients 4 --duration-ms 60000 --fire --timeout-ms 90000 --json-out bench/results/quake/multiplayer-browser-smoke-e1m7-4p-final-predeploy.json

Browser evidence

• deep E1M7: 6/6 checks passed; axe and shotgun damage A<->B; blood particles present; wall particles 0; pain/death frames present; reconnect passed
• 4-player 60s E1M7 soak: 0 rejects, 0 hidden/missing samples, max remote hidden/missing gap 0ms, 116 damage events, 20 kills