Artemis Milestone 2g (partial): bulk endpoint discovery + 2e disconnect teardown

Picked up from a crashed session: xhci_driver.h/xhci.h already had the
bulk_in/out_ep_addr/max_packet fields and Endpoint-descriptor offset
macros scaffolded, but the actual walk that populates them was never
written. Added it: after 2f confirms a Mass Storage/SCSI/BOT interface,
a nested walk continues through the Endpoint descriptors that follow it
(bDescriptorType==5, stopping at the next Interface descriptor or end
of stream), keeping only Bulk-type endpoints and splitting IN/OUT by
bEndpointAddress bit 7. Also reset the four new fields in
xhci_bringup(), which the scaffolding had missed.

Also completed 2e's disconnect teardown, which was fully implemented
this session (not scaffolded): a Disable Slot command is now submitted
on a real disconnect, with the port's tracked slot ID captured and
cleared from port_slot_id[] immediately (before the command completes)
so a fresh connect on the same port isn't confused for one already in
progress, and DCBAA[slot_id] cleared only on a successful completion.

Verified live via QMP hotplug (deliberate device_add/device_del against
freshly launched, individually-tracked instances -- not whatever
happened to be attached at boot), all three architectures,
byte-identical: bulk IN endpoint=0x81, bulk OUT endpoint=0x02, then a
clean disconnect -> disable slot succeeded, no wedge. Caught and fixed
a documentation near-miss in the same pass: an initial draft cited the
probe-free three-arch acceptance boots as this feature's verification
evidence, but a stale leftover log directory from a pre-crash orphaned
QEMU process had been picked up by an `ls -dt | head -1` glob during
monitoring and mistaken for this session's own result -- the real
acceptance logs never had a device attached at all. Re-verified against
real PIDs and real log paths before writing FABRIC-2.md's final
writeup.

FABRIC-2.md Section X Milestone 2 updated: 2e's disconnect-teardown
checklist item marked done, 2g's endpoint-identification item marked
partially done (identification only -- Configure Endpoint / EP Context
wiring is still open).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01R4VMX6VSKCten8nGgaMkq4

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