feat: F5.6d a drag is one change of the document; the others see it through the presence

Moving a device, moving or resizing a documentation item went into the
shared document at every pointer move: 59 updates a second, each leaving
about 18 bytes of history (M4 on test-filius). Now the editor keeps the drag
while it lasts, shows it itself and tells the others through the awareness
(moving, at most every 50 ms, the end at once), and writes it once when it
is let go. A drag the browser cancels or a second finger turns into a pinch
is written where it is; WebKit sends a second pointerdown for the first
finger when another comes, which had lost the drag.

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

packages/editor/browser/serverF.ts

@@ -33,6 +33,8 @@ export interface ServerF {
   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>;
+  /** How many updates of a project F has stored since it last compacted it. */
+  updateCount(project: string): Promise<number>;
   close(): Promise<void>;
 }
 
@@ -97,6 +99,10 @@ export async function startServerF(editorBase: string, editorFolder?: string): P
       const result = await sql.query<{ size: number }>('SELECT size FROM filius_blob WHERE sha = $1', [sha]);
       return result.rows[0]?.size;
     },
+    async updateCount(project) {
+      const result = await sql.query<{ count: string }>('SELECT count(*) FROM filius_doc_update WHERE project_uuid = $1', [project]);
+      return Number(result.rows[0]!.count);
+    },
     service: (method, path, body) => fetch(`${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));