chore(flow): expand host-return whitelist and add a DEBUG host bundle probe

- Rebuild the return-to-host whitelist from the CN/US top charts plus Apple
  system apps; drop the canOpenURL pre-check (LSApplicationQueriesSchemes
  is capped at 50) and let open() report failure.
- Never record an OSG bundle id as the host to return to.
- DEBUG only: probe the host bundle id through the keyboard arbiter
  callback and surface the log on a Home test card. Release builds keep
  the previous behaviour (no host identification).

OSGKeyboardExt/Services/KeyboardFlowCoordinator.swift

@@ -1138,6 +1138,7 @@ final class KeyboardFlowCoordinator {
         state.lastTranscript = ""
         recomputeMicVoiceAvailability()
         FlowSessionBridge.markPiPArmAttempt()
+        HostAppIdentity.publishReturnTarget()
         OSGDiag.log(
             "beginFlowStart → openHostApp(startflow) recordAfterHandoff=\(recordAfterHandoff) "
                 + "\(OSGDiag.memoryTag())",