Artemis Milestone 2h (foundational): sync wait bridge + SCSI READ CAPACITY(10)

Closes the gap block_subsystem.c needs before any of 2h's real work
(blkio_usb.c, attach wiring, hot-detach) can start: this driver is fully
async/polled with no way for a synchronous caller (blkio_read()/
blkio_info() etc.) to get a result back. xhci_bot_wait_for_idle() is a
bounded busy-wait over xhci_poll_events() -- MUST be called only from
outside xhci_poll_events()'s own call frame, never from within it or a
next_action dispatch (recursion into live Event Ring/ERDP processing,
same class of hazard already documented for doorbell rings in this
driver). xhci_get_dev() exposes the module-static device handle to
outside callers that didn't observe the original hotplug event.

SCSI READ CAPACITY(10) (opcode 0x25) is the other half -- nothing could
learn a device's block size/capacity before this. First attempt sent it
bare and hit the classic first-command UNIT ATTENTION (CSW FAILED); fixed
with the same TUR-guard pattern READ(10) already used, generalized via a
new bot_tur_chain_target field so TEST UNIT READY's PASS handling can
chain into either command. bot_data_buf grown 512->1024 bytes (one Forth
block = two 512-byte SCSI blocks, per block_subsystem.c's own 1KiB-unit
convention).

Verified live via a temporary probe (hot-attached disk/usb-thumbdrive-
test.img via QMP, reverted after capture): TUR-guarded READ CAPACITY10
correctly reported last LBA=0x1ffff, block size=0x200 -- exactly 64MiB,
matching the test image byte for byte -- followed by a TUR-guarded
1024-byte/2-block READ10, both PASS. All three architectures re-verified
clean, probe-free boot to ok> on the reverted tree. FABRIC-2.md Section X
2h updated with the writeup; the blkio_usb.c backend itself is next.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CXjAPTEKrgY2Mrk25KoLDn

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