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/server/src/simChannel.ts

@@ -4,10 +4,16 @@
 // 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).
+// numbers them). A room's large binary files (blobs) are F's: the thread
+// asks for the ones it has not got, and gives F the ones the simulation
+// makes before the document's change that names them.
 
 import type { HostMessage } from '@mdg-filius/protocol';
 
+/** How often, and how long apart, a thread asks again for a blob F has not got (yet). */
+export const BLOB_ASKS = 5;
+export const BLOB_ASK_AGAIN_MILLISECONDS = 1_000;
+
 export type ToRoomThread =
   /** A room for a project, its document as it is now. */
   | { readonly type: 'open'; readonly room: string; readonly state: Uint8Array }
@@ -19,10 +25,16 @@ export type ToRoomThread =
   /** 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 };
+  | { readonly type: 'close'; readonly room: string; readonly writeBack: boolean }
+  /** The blobs asked for: those F has, and the names of those it has not. */
+  | { readonly type: 'blobs'; readonly room: string; readonly blobs: readonly { readonly sha: string; readonly bytes: Uint8Array }[]; readonly missing: readonly string[] };
 
 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 }
+  /** Blobs the document names that the thread has not got. */
+  | { readonly type: 'needBlobs'; readonly room: string; readonly shas: readonly string[] }
+  /** A blob the simulation made (before the change that names it). */
+  | { readonly type: 'blob'; readonly room: string; readonly bytes: Uint8Array }
   | { readonly type: 'closed'; readonly room: string };