Implement homeblocks_sig_check(): the drive signature check (Phase 8)

Real, complete verification logic -- not yet wired to any write path.
homeblocks_sig_check(dev, sig_start_fblock, out_sig) reads the 4
consecutive 1KB blkio forth-blocks the 4KB header spans, verifies
magic -> version -> CRC-64 in order, returns HOMEBLOCKS_SIG_OK/_BLANK/
_BAD_VERSION/_BAD_CRC/_READ_ERROR. Reuses block_subsystem.c's existing
CRC-64/ISO (compute_crc64, previously static/file-local, now exposed
via block_subsystem.h) rather than a second CRC implementation --
same algorithm already proven via per-block checksums. Takes the
header's starting block as a plain parameter rather than resolving it
internally: verifies a signature given a location, finding that
location (GPT-partition-relative) stays the caller's job.

Verified against the actual shipped code, not a reimplementation: a
standalone host test links the real homeblocks_sig.c against a fake
in-memory blkio_dev and exercises all four outcomes -- blank media,
a correctly-minted header (round-trips drive_uuid/minted_time_ns), a
flipped CRC, an unrecognized version. All four pass. A full
QEMU-hotplug live test isn't proportionate yet since nothing calls
this function from the live kernel path -- wiring it into the attach
path is the next punch-list item. Clean zero-warning compile and
clean boot on all three architectures confirms no build/link
regression from exposing compute_crc64 and adding the new source
file to every kernel build.

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

src/block_subsystem.c

@@ -104,7 +104,7 @@ static void crc64_init(void) {
     crc64_inited = 1;
 }
 
-static inline uint64_t compute_crc64(const uint8_t *data, size_t len) {
+uint64_t compute_crc64(const uint8_t *data, size_t len) {
     if (!crc64_inited) crc64_init();
     uint64_t crc = 0xFFFFFFFFFFFFFFFFULL;
     for (size_t i = 0; i < len; i++) {