fix(tiles): a tile that joined before its pane existed resends its size

A session Run makes while the grid is open joins as a tile right away, so
the tile connects and sends its size before Run starts the pane. The
server only records a resize for a session with no PTY and spawns the pane
at 120x40, and Run's own resize step measures the parked main terminal
(display: none, so nothing). Measured live for Shell and Claude: a 97x17
tile over a 120x40 pane, for good (#464).

The chrome refresh now remembers each tile's last-seen pid and calls
TerminalTile.paneStarted() when it appears or changes. paneStarted()
forgets the sent size and sends it; a hidden tile (a zoomed neighbour)
sends nothing and keeps it forgotten, so its next fit() sends it, which a
plain fit({ force: true }) would lose. Keyed on the sessions map, so a
handleInit after an SSE drop counts too.

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

test/mocks/tile-grid-vm.ts

@@ -134,6 +134,7 @@ export class FakeTile {
   reconnectNow = vi.fn();
   fit = vi.fn();
   localFit = vi.fn();
+  paneStarted = vi.fn();
   destroy = vi.fn(() => {
     this._destroyed = true;
   });