fix(host): cancellable listing scans and O(log window) insertion

capability.list gains an optional AbortSignal threaded from the RPC
carrier's request signal (the pickDirectory pattern): a disconnected or
timed-out caller stops the opendir loop instead of the scan outliving
its caller, and the abort surfaces as its own reason rather than a
directory-unreadable dressing. boundedInsert rejects a full window's
at-or-beyond-tail candidate on one comparison and binary-inserts
retained candidates, so an oversized level no longer pays a window scan
per dirent.

packages/client/connection/src/client/fixture.ts

@@ -1160,7 +1160,7 @@ export class FixtureApiClient extends AbstractApiClient {
       case 'session.cancel': return this.api.sessions.cancel(request)
       case 'host.describe': return this.api.host.describe(request)
       case 'host.pickDirectory': return this.api.host.pickDirectory(request, new AbortController().signal)
-      case 'host.listDirectory': return this.api.host.listDirectory(request)
+      case 'host.listDirectory': return this.api.host.listDirectory(request, new AbortController().signal)
       case 'host.createDirectory': return this.api.host.createDirectory(request)
       case 'host.openPath': return this.api.host.openPath(request, new AbortController().signal)
       case 'workspace.list': return this.api.workspace.list(request)