Artemis Milestone 2h: blkio_usb.c backend -- USB thumb drive is now a real block device

Wires a hot-plugged USB Mass Storage device into the block subsystem's
unified LBN chain. blkio_usb.c/blkio_usb.h mirror virtio_blk.c/
virtio_blk.h's established shape exactly (singleton state, blkio_vtable_t,
a blkio_usb_open_msc() "find" function playing virtio_blk_find_artemis()'s
role): read() translates a Forth block into a SCSI LBA/count pair and
calls xhci_bot_read_block() + xhci_bot_wait_for_idle(); write() returns
BLKIO_ENOSUP (no SCSI WRITE(10) exists yet, and blk_format_or_load_disk()
never writes at attach time, so read-only is sufficient -- confirmed by
reading that function first, not assumed). Refuses (-2) if the reported
SCSI block size doesn't evenly divide the 1024-byte Forth block size.

Connect-time wiring reuses the bot_msc_attach_pending/consume-in-
sk_repl_idle() shape the prior increment's temp probe already validated,
now made permanent: SET_CONFIGURATION sets the flag, sk_repl_idle()
(strictly after its own xhci_poll_events() call returns) calls
blkio_usb_open_msc() then blk_subsys_attach_device().

Verified live via hot-attach: full chain from USB connect through
'blkio_usb: MSC device ready' to 'blk: disk 'StarForth Volume' v2 LBN
26074..75184 (49111 user blocks)' -- real attachment, disk image confirmed
byte-for-byte untouched after. Chased a real debugging detour along the
way: the attach initially appeared silent (no blk: log line) -- traced to
LOG_INFO filtering at the default LOG_WARN boot level, not a functional
bug (settled via a temporary log-level bump, reverted after capture; also
found and reported, but did not fix, a pre-existing unrelated
Makefile.starkernel bug where --log-level=info via KERNEL_ARGS breaks
printf parsing). All three architectures re-verified clean. FABRIC-2.md
Section X 2h updated -- only hot-detach remains for 2h.

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

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