test: wait for the first read before starting the other edit history sessions

The gate belonged to whichever read arrived first, and sessions load their stores
independently, so under CI timing another session took it, the loop never reached
release() and the test timed out (master, CI run 41930703100). The code cannot block
one session on another: sync() only waits on its own slot.

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

test/edit-history-sync.test.ts

@@ -428,14 +428,20 @@ describe('EditHistorySync', () => {
     host.append('main', editLine('t1', T0, '/w/a.ts'));
     const orig = host.read;
     let first = true;
+    let inRead!: () => void;
+    const reachedRead = new Promise<void>((r) => (inRead = r));
     host.read = async (...a) => {
       if (first) {
         first = false;
+        inRead();
         await gate;
       }
       return orig(...a);
     };
     const slow = sync.sync(local);
+    // Only once s1 holds the gate: the sessions load their stores independently, so a later one
+    // could reach the read first, take the gate and hang the loop below before `release()`.
+    await reachedRead;
     for (let i = 1; i <= MAX_LOADED_STORES + 1; i++) await sync.sync({ ...local, id: `s1-${i}` });
     release();
     expect(await slow).toEqual({ state: 'ok' });