Stage MC-0b2: block subsystem behind g_lock_block (FABRIC-3.10.8 §3.3)

- block_subsystem.c (vendored): every public function's body becomes a
  static *_locked function; bodies call each other only through the
  _locked forms, breaking the internal cycles (relocate, meta relocate,
  migration scan, firsttouch, the meta accessors). Thin public wrappers
  take g_lock_block under __STARKERNEL__; hosted build has no lock and
  still builds clean.
- Lock order: hermes, switch, registry, doe, log, block, console, heap,
  vmm, pmm (Stadium, MC-0d, above block).
- Documented, not coverable by this lock: blk_get_buffer() pointers are
  used after release -- block-buffer ownership across cores is open for
  MC-3.
- Probe (removed): block lock balances on all three ISAs (2,057-2,379
  acquisitions), 0 recursions, 0 held at switch; vblk_io timeouts
  (0/0/4 probe, 0/0/2 acceptance) within the pre-lock riscv64 baseline
  of 0-6.
- Standard sequence on amd64/aarch64/riscv64: hashes and G byte-identical
  to MC-0b, MINT/WIREBIND/USE ok, conservation true, 0 idle growth.
- FABRIC-3.10.8 §3.2 wording fix; lock order noted as checked by
  inspection only.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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