Artemis Milestone 2f: EP0 control transfer, device descriptor request

Adds Setup/Data/Status stage TRB types and control bits (IDT, TRT, DIR)
to xhci.h, and xhci_ep0_enqueue_trb()/xhci_ep0_get_device_descriptor() to
xhci.c -- the first real control transfer this driver has issued.
Follows the same enqueue-then-doorbell-once pattern as the Command Ring,
operating on the EP0 Transfer Ring built during 2e's Address Device work.
Setup Stage uses Immediate Data (parameter IS the 8-byte setup packet);
Data Stage reads into a reused 18-byte device_descriptor buffer; Status
Stage alone carries IOC, so exactly one Transfer Event signals transfer
completion, correlated via a new pending_transfer_slot_id (same
single-outstanding-operation pattern as connect/Enable Slot/Address
Device).

Automatically triggered once Address Device succeeds. Verified live via
QMP hotplug, all three architectures, worked first try with identical
results everywhere: idVendor=0x46f4, idProduct=0x0001, bDeviceClass=0x00
-- the class=0 confirms Mass Storage class detection needs the
Configuration/Interface descriptor (2f's next item), not the device
descriptor.

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

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