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/editor/browser/serverF.ts

@@ -29,6 +29,8 @@ export interface ServerF {
   /** Signs a person in, in this browser context. */
   signIn(context: BrowserContext, project: string, person: { readonly name: string; readonly level?: 'read' | 'write'; readonly user?: string }): Promise<void>;
   service(method: string, path: string, body?: unknown): Promise<Response>;
+  /** The size of a blob in F's store, none without it. */
+  blobSize(sha: string): Promise<number | undefined>;
   close(): Promise<void>;
 }
 
@@ -88,6 +90,10 @@ export async function startServerF(editorBase: string): Promise<ServerF> {
       if (response?.status() !== 200) throw new Error(`not signed in: ${response?.status()}`);
       await page.close();
     },
+    async blobSize(sha) {
+      const result = await sql.query<{ size: number }>('SELECT size FROM filius_blob WHERE sha = $1', [sha]);
+      return result.rows[0]?.size;
+    },
     service: (method, path, body) => fetch(`${base}${path}`, { method, headers: { 'x-filius-service-key': SERVICE_KEY }, ...(body === undefined ? {} : { body: JSON.stringify(body) }) }),
     async close() {
       const ended = new Promise((resolve) => child.once('exit', resolve));