feat: F5.5 large binary files as blobs on server F

From 16 KB a picture's or binary file's entry names its bytes by their
SHA-256 instead of holding them; the bytes are on F, content-addressed
(GET/POST /blobs with the session's grant, from the editor's origin, at
most 16 MiB). A document's blob side says where they are: the editor's
fetches and uploads, the room thread's asks F's main thread, and a start
waits for the ones asked for; without a side everything stays in the
document. Text stays Y.Text. A blob whose bytes are not there is never
written back empty. A blob F has not got yet is asked for again a few
times, and a running room reads late bytes.

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

packages/sim/src/room.ts

@@ -191,6 +191,15 @@ export class SimulationRoom {
     }
   }
 
+  /**
+   * The bytes of blobs came (on F, after the simulation read the files
+   * without them): every node's files read anew into the running engine.
+   */
+  blobsArrived(): void {
+    if (this.mode !== 'simulation') return;
+    this.guarded(() => this.syncFiles(new Set(this.engine.state.topologySnapshot.fileSystemsByNodeId.keys())));
+  }
+
   /** Whether a node's files changed since they were last written back; marked written if so. */
   private takeChangedFiles(id: Id, files: { readonly revision: number } | undefined): boolean {
     const written = this.writtenFiles.get(id);