USB: P2 crossbeam bounded channels — prevent OOM under USB load

IMPROVEMENT-PLAN.md §10.2 item 4: medium priority fix.

Changed two crossbeam channels from unbounded to bounded:
- irq_reactor: 1024 events (transfer/command completions)
- device_enumerator: 64 events (port enumeration requests)

Unbounded channels can grow without limit if the consumer
(IRQ reactor) falls behind, causing OOM under heavy USB traffic.
Bounded channels provide natural backpressure — the sender
(scheme handler) blocks when the channel is full, causing
the USB client to back off.

Cross-referenced with Linux 7.1 xhci-ring.c producer/consumer
pattern where transfer rings are bounded by hardware limits.

drivers/usb/xhcid/src/xhci/mod.rs

@@ -467,9 +467,9 @@ impl<const N: usize> Xhci<N> {
         let entries_per_page = PAGE_SIZE / mem::size_of::<Trb>();
         let cmd = Ring::new::<N>(ac64, entries_per_page, true)?;
 
-        let (irq_reactor_sender, irq_reactor_receiver) = crossbeam_channel::unbounded();
+        let (irq_reactor_sender, irq_reactor_receiver) = crossbeam_channel::bounded(1024);
 
-        let (device_enumerator_sender, device_enumerator_receiver) = crossbeam_channel::unbounded();
+        let (device_enumerator_sender, device_enumerator_receiver) = crossbeam_channel::bounded(64);
 
         let mut xhci = Self {
             base: address as *const u8,