perf(mobile): open a session with parallel startup RPCs and a pre-warmed terminal engine (#19260)

Startup RPCs now fan out in parallel and the xterm engine pre-warms inside the
real terminal frame while they are in flight, so the first pane inherits a warm
WebView and an already-measured viewport instead of paying a round trip for it.

The pre-warm opens its engine before measuring: web-ready only reports that the
bundle loaded, and the WebView answers a measure with null until a terminal
exists. It also pre-warms at the user's saved text size, because cell size is
what the frame height gets divided by.

Host writes such as worktree.activate wait for an evaluated status.get reply.
Navigation still fails open when a host cannot answer one, but that fallback no
longer reads as a passing compatibility verdict.

mobile/src/worktree/home-host-worktree-fetch.ts

@@ -13,7 +13,7 @@ import { WORKTREE_PS_FULL_LIMIT } from './worktree-catalog-snapshot-client'
 const ACTIVE_STATUSES = new Set(['working', 'active', 'permission'])
 // Why: a relay↔direct cutover rejects in-flight reads without ever leaving 'connected', so the
 // connect gate never re-arms. Re-issue on the replacement session; cap it so a migration loop
-// can't spin. See runtime-capability-probe.ts for the same hazard on status.get.
+// can't spin. See runtime-status-probe.ts for the same hazard on status.get.
 const CUTOVER_RETRY_LIMIT = 2
 
 export type HostWorktreeInfoSetter = (