perf(tiles): opening paints the tiles first, one terminal per frame after

Owner answer: "paced connect: in". Measured on the checkpoint build: of
the 140 ms between the click on Tiles and its first frame, about 100 ms
was the six xterms being built inside the click, so nothing moved on
screen for that long and the entrance could not start.

openTileGrid now mounts every tile and lays the grid out as before, then
_connectTilesPaced builds one terminal per animation frame, the focused
tile's first, then reading order. The click paints its empty tiles in
about 20 ms and the entrance plays while the terminals are built. The
time until every tile has painted does not change: the load queue serves
one capture at a time, so only the focused tile's connect is on its path,
one frame later. Each tile still connects once, into its final cell (one
fit, one PTY resize).

- openTileGrid returns before the terminals exist, so a selection that
  focuses a tile whose terminal is not built yet hands the keyboard over
  in _connectTile (focusOnConnect), never when focus: false was asked,
  and to the newly focused tile when focus moved meanwhile.
- _connectTile connects a tile once (entry.connected; a remount after
  Attach resets it), so a re-form or a remount before a tile's turn is
  never connected twice.
- A grid closed or opened again meanwhile stops the old run (a run
  token), and asks for no further frames.

Tests: tile-grid-paced-connect.test.ts (the order, the final cells, the
keyboard, close and reopen, removed and remounted tiles); the tests that
read connect or the terminal's focus right after openTileGrid now run
the queued frames first (flushFrames in the vm harness), every
assertion kept.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test/tile-grid-dividers.test.ts

@@ -17,6 +17,7 @@ import {
   FakeEl,
   FakeTile,
   body,
+  flushFrames,
   makeGridApp,
   rafCallbacks,
   resetGridHarness,
@@ -32,6 +33,8 @@ const tile = (id: string) => FakeTile.all.find((t) => t.sessionId === id) as Fak
 function grid(ids: string[]): GridApp {
   const app = makeGridApp(ids);
   app.openTileGrid(ids);
+  // The tiles' terminals are built one per frame (_connectTilesPaced).
+  flushFrames();
   for (const t of FakeTile.all) {
     t.fit.mockClear();
     t.localFit.mockClear();