feat: F5.3a the simulation rooms on server F

One room per project for everyone in it, held by a few worker_threads,
each room with a replica of the document that Hocuspocus' direct
connection keeps open: changes go in, what the room writes comes back.
/sim?project=... signs in as /sync does and says refusals and partings
as the socket's closing; only a write grant changes the room. The state
names who started the simulation. After the last person the room stays
60 s, then stops, writes back and ends. Freezing stops a running
simulation and refuses to start one, deleting ends the room without
writing back, revoking lets the person go from the room too; a failed
thread ends its rooms, stopping the service stops them all.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/server/src/simChannel.ts

@@ -0,0 +1,28 @@
+// The messages between server F's main thread and a thread that holds
+// simulation rooms (plan §8.2: rooms in worker_threads, so that a large
+// network does not hold up the shared documents). A thread holds several
+// rooms, each with a replica of its project's document: the document's
+// updates go both ways, a person's messages go in, the room's come out,
+// each for one person (a room tells its people apart; the main thread
+// numbers them).
+
+import type { HostMessage } from '@mdg-filius/protocol';
+
+export type ToRoomThread =
+  /** A room for a project, its document as it is now. */
+  | { readonly type: 'open'; readonly room: string; readonly state: Uint8Array }
+  | { readonly type: 'doc'; readonly room: string; readonly update: Uint8Array }
+  /** A person in the room, by name. */
+  | { readonly type: 'connect'; readonly room: string; readonly client: number; readonly name: string }
+  | { readonly type: 'client'; readonly room: string; readonly client: number; readonly message: unknown }
+  | { readonly type: 'disconnect'; readonly room: string; readonly client: number }
+  /** The simulation stopped (what it left written back first); answered by `stopped`. */
+  | { readonly type: 'stop'; readonly room: string }
+  /** The room ended — stopped first with `writeBack`, else as it is (its project is gone); answered by `closed`. */
+  | { readonly type: 'close'; readonly room: string; readonly writeBack: boolean };
+
+export type FromRoomThread =
+  | { readonly type: 'doc'; readonly room: string; readonly update: Uint8Array }
+  | { readonly type: 'host'; readonly room: string; readonly client: number; readonly message: HostMessage }
+  | { readonly type: 'stopped'; readonly room: string }
+  | { readonly type: 'closed'; readonly room: string };