Artemis Milestone 2g: TEST UNIT READY unit-init sequence -- READ(10) now PASSes

Roots out the CSW status FAILED left unexplained in the prior increment: a
freshly attached SCSI target's standing UNIT ATTENTION condition, which a
bare READ(10) with no retry can never clear. xhci_bot_send_test_unit_ready()
sends SCSI TEST UNIT READY (SPC-4 6.33) ahead of the real command; the CSW
handler now tags command kind (bot_cmd_kind) to distinguish a TUR completion
from a READ10 completion, chains TUR PASS into the real READ(10), and
bounded-retries TUR on FAILED/PHASE ERROR (bot_tur_retries, capped at
XHCI_BOT_TUR_MAX_RETRIES). xhci_bot_read_block() is the new intended entry
point tying lba/num_blocks/block_size + the TUR-first sequencing together.

Verified live via a temporary probe (hot-attached disk/usb-thumbdrive-test.img
through the running instance's QMP socket), captured on amd64: full chain
CBW(TUR) -> FAILED -> retry -> PASS -> CBW(READ10) -> Data-In -> CSW PASS.
Probe reverted after capture; all three architectures re-verified clean,
probe-free boot to ok>. FABRIC-2.md Section X 2g updated with the writeup.

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

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