Artemis Milestone 2e: PORTSC connect/disconnect detection, verified live

xhci_poll_events()'s Port Status Change branch now decodes the Port ID
from the event TRB (XHCI_PSC_EVT_PORT_ID, new in xhci.h), reads that
port's PORTSC.CCS via a new xhci_port_regs() helper, and logs connect vs.
disconnect. Acknowledges by writing back only PP (preserved) and CSC (the
bit being cleared) -- PED/PR/other _C bits written 0 so nothing is
accidentally disabled, reset, or silently cleared, matching the RW1C
discipline already used for ERDP.EHB in 2d.

Verified with the real target scenario via QMP hotplug on all three
architectures: boot with the xHCI controller present but no USB device
attached (confirmed zero port activity at ok>), then live
attach/detach/re-attach of a virtual USB thumb drive
(disk/usb-thumbdrive-test.img via usb-storage on xhci0.0). Full
connect->disconnect->connect cycle confirmed clean (no port wedge) on
amd64; single connect confirmed on aarch64 and riscv64.

Still open: correlating Command Completion Events back to their issuing
command, driving Enable Slot/Address Device from this connect path
(currently only a boot-time smoke test), and the callback surface into
Section U's higher-level code.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HZ8kNoTuP63pbQtro4qvrm

include/starkernel/xhci_driver.h

@@ -78,10 +78,12 @@ int xhci_bringup(xhci_dev_t *dev);
 
 /*
  * xhci_poll_events — read Interrupter 0's Event Ring, dispatching each TRB
- *                     by type (Port Status Change, Command Completion;
- *                     other types logged and skipped -- Milestone 2e/2g
- *                     consume them), then advance the Event Ring dequeue
- *                     pointer and clear ERDP.EHB.
+ *                     by type: Port Status Change reads PORTSC to log
+ *                     connect/disconnect and acknowledges CSC; Command
+ *                     Completion and Transfer Event are logged only (slot
+ *                     allocation and BOT transfers are later increments).
+ *                     Advances the Event Ring dequeue pointer and clears
+ *                     ERDP.EHB when done.
  *
  * Polled, not interrupt-driven: an initial attempt at IRQ delivery
  * (Milestone 2d's first draft) found the amd64 PCI INTx routing formula