wip: async BT firmware download attempt — DO NOT MERGE, breaks AX211 bring-up

Deferring the Intel BT firmware download off the boot path made the AX211
bootloader stop answering after the first FC05; even the final synchronous
revert freezes boot, so one of the 'neutral' fixes kept in this diff breaks
the bring-up on its own (candidates: BT-TRACE logging inside TransferCallback,
unconditional interrupt-IN re-queue after error completions on a halted EP,
xHCI interrupt-IN ZLP len fix interacting with HID, InPollContext owner
check).  Full history + next experiments in memory notes, 2026-07-05/06.

Co-Authored-By: Claude Fable 5 <[email protected]>

kernel/src/Timekeeping/ApicTimer.cpp

@@ -212,6 +212,13 @@ namespace Timekeeping {
             Drivers::USB::Xhci::ProcessDeferredWork();
         }
 
+        // Complete any boot-deferred Bluetooth bring-up (Intel firmware
+        // download) here instead of on the boot path: the download takes
+        // seconds and used to stall kmain before the first process spawned.
+        // Cheap no-op unless an adapter is waiting; self-claiming, and safe
+        // to preempt (the scheduler saves/resumes the idle context).
+        Drivers::USB::Bluetooth::ServiceDeferredInit();
+
         // Service Bluetooth inbound traffic (the headset's SDP/AVRCP queries,
         // AVDTP commands, ACL flow-control credits) whenever a core idles.
         // Without this, BT events are only processed while some syscall happens