Wire homeblocks_sig_check() into USB attach: the warn half (Phase 8)

Wired into sk_repl_idle()'s USB hotplug attach handler, right between
blkio_usb_open_msc() succeeding and blk_subsys_attach_device() -- logs
a distinct message per outcome (recognized / blank-or-foreign /
bad-version / bad-crc / read-error).

The "refuse" half is deliberately not implemented -- there is nothing
real to gate yet. blkio_usb.c has no SCSI WRITE(10) support at all, so
there is no write path today to refuse; attach currently only enables
read-only access, which is also the general-purpose USB block I/O
path this repo already relies on for unrelated testing, not
exclusively a home-blocks identity workflow. Refusing attach on blank
media would break that legitimate use without protecting anything
real -- same "don't build ahead of a real caller" reasoning EXPIRE's
deferral used. Refuse belongs on the write path once WRITE(10) exists.

sig_start_fblock is hardcoded to 0 at the call site -- correct for
today's unpartitioned raw test/real media (no GPT parser exists yet),
flagged in the comment as the one place that changes once a real
GPT-partition-relative lookup exists, isolated from
homeblocks_sig.c's own location-agnostic check logic.

Verified live: hot-attached disk/usb-thumbdrive-test.img (blank media)
through a running amd64 instance's QMP socket (blockdev-add +
device_add usb-storage) -- captured exactly 4 real TUR+READ10 BOT
cycles (matching the header's 4 forth-block span) followed by the
correct "not recognized" warning, then normal attach completing
successfully afterward (no regression). Conservation intact, no
panic. Clean zero-warning compile and clean boot on all three
architectures.

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

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