Add custom gameplay rooms and pluggable client scenes

[RSamaium]

Summary

• add a public gameplay-room registry, decorators, providers, and RpgGameplayRoom base class
• let players move between map, lobby, and custom rooms while preserving their authoritative identity and variables
• add pluggable client scenes for custom room kinds without mounting the map viewport or physics pipeline
• preserve the existing map/lobby lifecycle and protocol behavior
• document runtime ownership, standalone/MMORPG behavior, CanvasEngine .ce scenes, private projections, cleanup, and reusable module boundaries
• include the required beta changeset and map-return streaming fixes

Why

RPGJS v5 already had useful room primitives internally, but the public server and client runtimes still assumed that gameplay happened in map or lobby rooms. This prevented battle, card-game, matchmaking, and similar modes from using a dedicated synchronized room and client scene without depending on Tiled maps and the map rendering pipeline.

This introduces a stable room descriptor and registry shared across the runtime. Custom room state and player snapshots remain server-authoritative and synchronized, while each registered room kind can select its own CanvasEngine scene on the client.

Closes #347.

Validation

• pnpm build — all 12 packages
• pnpm test:types — 4 files, 30 tests
• pnpm api:boundary — 27 public entry points checked
• pnpm vitest run packages/common/src packages/client/src packages/server/tests packages/testing/src — 81 files, 692 passed, 16 skipped
• RFC RFC: introduce entity rooms and pluggable client scenes beyond maps #347 PoC room-flow integration — 1 passed
• RFC RFC: introduce entity rooms and pluggable client scenes beyond maps #347 PoC production builds — standalone and MMORPG passed
• generated Player and Client API pages
• git diff --check
• jq empty docs/docs.json

The Mint documentation CLI is not declared or installed in the local docs workspace, so pnpm --dir docs validate could not run locally; GitHub CI remains the source of truth for that check.