Stage MC-0b: real kernel spinlocks; sf_mutex no longer a no-op (FABRIC-3.10.8 §3.2)

- New spinlock primitive (spinlock.h/.c): CAS on an owner word (core+1),
  interrupts masked while any lock is held (state kept in the per-core
  block, so release order doesn't matter). Recursion, release-not-held,
  held-at-switch and destroyed-while-held are counted and logged, never
  silent. Locks are no-ops until the boot core's block is installed
  (riscv64's monolithic early boot prints before sk_percpu_boot_init()).
- Kernel locks in a documented order: hermes, switch, registry, doe, log,
  console, heap, vmm, pmm. sf_mutex (dict_lock/tuning_lock) uses the same
  lock word.
- Hermes: public functions over *_locked internals; never held across the
  drain's vm_interpret_kind(); switch notifications after dropping the
  lock; event payload moved off static storage. Switch table and registry
  never call out while held (slot/entry copied under the lock).
- No lock crosses a switch: checked at context switch (leave, resume) and
  the idle point.
- Probe (removed): every lock balances on all three ISAs; 0 held at
  switch over 7.5M-51M checks; 0 recursions, 0 bad releases.
- Standard sequence on amd64/aarch64/riscv64: hashes and G byte-identical
  to MC-0a, MINT/WIREBIND/USE ok, conservation true, 0 idle growth.
- Block subsystem split out as MC-0b2 (Captain Bob's ruling).

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

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