Keep shared membership per client session and add helpers.ensureJoined

[Noisemaker111]

An LLMville player reported "it says I can place, but it fails". Shared-world membership is stored per user, but clients join and leave per session. Closing a second tab, or a join that resolves after a remount (StrictMode, quick route changes), evicted the user while the open tab still showed "joined". Every host mutation then failed with "Not a member of this server".

• The core transport's joinServer/leaveServer take an optional sessionId, and createTransportSessionId() generates one.
• Convex shared topology:
  • Each member row keeps up to 16 live sessionIds.
  • A session leave removes only its own id, and the full leave runs only when the last id goes.
  • A leave for an id that isn't live does nothing.
  • A leave with no id, and legacy rows, behave as before.
• helpers.ensureJoined(ctx, { gameId, userId, serverId? }) runs the join path for an actor host code has already authorized. With serverId set and the user already a member, it returns without extra work. runCommand docs now point to it.
• useServerSession, the Convex transport's pagehide leave, and the shell sync each pass one id per session.
• ws: join and leave messages carry sessionId. A user is only removed when no other connection of theirs still holds the server.
• The jgengine-multiplayer skill covers sessions and ensureJoined.

Bugfix notes

• Root cause: membership was per user while join and leave were per session.
• Any game on the shared Convex topology, or on ws with singleSession: false, can hit this.
• Still open: rooms topology still leaves per user. Fixing it needs a new field on jgGameServers.

Verification

• bun test packages/convex packages/core/src/runtime packages/react packages/ws packages/shell: 1355 pass, 0 fail.
• New tests cover two sessions, stale leaves, legacy rows, the 16-id bound, ensureJoined idempotence, the pagehide transport, and ws multi-connection.
• check-types for core, convex, react, ws and shell: pass.
• bun run gen: done; ship:preflight: ok.
• There's no rendered surface to screenshot.

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https://claude.ai/code/session_01GvRbbu8yG1WYz7wQnZUpC8

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