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 <noreply@anthropic.com>

kernel/src/Hal/SmpBoot.hpp

@@ -40,6 +40,14 @@ namespace Smp {
         Hal::TSS64* tss;          // pointer to this CPU's TSS
         bool hasMwait;            // CPU supports MONITOR/MWAIT
 
+        // When set, the scheduler tick must NOT pull this (idle) CPU away to
+        // run a ready process: its idle context is busy with long-running
+        // kernel work whose wall-clock-bounded waits would spuriously expire
+        // if the context only ran when nothing else is ready (observed: the
+        // deferred Bluetooth firmware download racing login-screen startup).
+        // Other CPUs schedule normally while this is held.
+        volatile bool reservedForKernelWork = false;
+
         // Per-CPU GDT and TSS (APs use these; BSP uses globals)
         Hal::BasicGDT cpuGdt __attribute__((aligned(16)));
         Hal::TSS64    cpuTss __attribute__((aligned(16)));