feat(tiles): one load queue for every capture a grid tile fetches

GET /api/sessions/:id/terminal runs synchronous tmux calls on the server, so
N tiles loading at once would stall every WebSocket and SSE stream back to
back (and after a deploy restart all N reopen within the same second).

TerminalTile takes the options PR 1 deferred to the grid:

- scheduleLoad(tile, kind, run): every capture the tile fetches (initial
  load, reconnect refresh, server {t:'r'} refresh, shell history pull) runs
  when its owner says so. Absent (the split's Pane B), a load runs at once.
- scrollback (the grid passes TILE_SCROLLBACK) and fontSize.
- boundedLoad: a TUI tile loads the bounded full=1&tail= window, never its
  whole history.

TileLoadQueue (terminal-tile.js, DOM-free) is that one queue: concurrency 1,
a history pull ahead of background refreshes, then the owner's rank (the grid
ranks the focused tile first, then reading order). A destroyed tile's waiting
loads are dropped unrun, and destroy() aborts the running fetch so the queue
moves on.

Also, for Pane B as well: the load now has a deadline covering the body
(CodemanFetchDeadline), so a capture that never answers cannot hold the
single-flight flag (or the queue) forever; a refresh clears the screen at its
turn rather than when it is asked for; and a close while a load only waits in
the queue writes the disconnected marker at once.

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

test/terminal-tile-unit.test.ts

@@ -221,7 +221,8 @@ describe('TerminalTile server-refresh single-flight', () => {
     await settle();
 
     expect(pane.terminal.clear).toHaveBeenCalledTimes(1);
-    expect(fetchMock).toHaveBeenCalledWith('/api/sessions/s1/terminal?full=1');
+    // The second argument carries the load's deadline (an AbortSignal).
+    expect(fetchMock).toHaveBeenCalledWith('/api/sessions/s1/terminal?full=1', expect.anything());
     expect(pane.terminal.write).toHaveBeenCalledWith('one');
     expect(pane._bufferLoading).toBe(false);
   });
@@ -233,7 +234,7 @@ describe('TerminalTile server-refresh single-flight', () => {
     pane._refreshBuffer();
     await settle();
 
-    expect(fetchMock).toHaveBeenCalledWith(`/api/sessions/s1/terminal?tail=${1024 * 1024}`);
+    expect(fetchMock).toHaveBeenCalledWith(`/api/sessions/s1/terminal?tail=${1024 * 1024}`, expect.anything());
   });
 
   it('refreshes arriving mid-fetch neither clear nor fetch again, and run ONCE after the replay lands', async () => {