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/Sched/Scheduler.cpp

@@ -945,7 +945,12 @@ namespace Sched {
             // scanning 256 process slots on every tick. On a 32-core
             // system with 27 idle CPUs, this avoids ~7M cache-line
             // reads/sec from the process table.
-            if (readyCount > 0) {
+            //
+            // A CPU whose idle context is reserved for long-running kernel
+            // work (e.g. the deferred Bluetooth firmware download) is left
+            // alone: stealing it starves that work of CPU time while its
+            // wall-clock timeouts keep running.
+            if (readyCount > 0 && !cpu->reservedForKernelWork) {
                 Schedule();
             }
             return;