starkernel: document the mutex stub's concurrency constraint (item 0.9)

sf_mutex_lock()'s no-op is correct only while nothing in interrupt
context mutates the structure a given lock protects. States that
explicitly, and states why a real spinlock is the wrong fix if that
constraint is ever violated: a single hart would deadlock spinning
against its own interrupt handler. The correct fix in that case is
disabling interrupts around the critical section.

Documentation only, no behaviour change.

Punch list §25 item 0.9 complete.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

FABRIC.md

@@ -2150,7 +2150,7 @@ on until there is a tick on all three architectures (§16.1, §16.5).*
   > DoE run in a later phase or a dedicated look at the warm-up gate.
   *Refs:* §16.4 (as ruled), §18.4, §21.2, §26.
 
-- [ ] **0.9 — Write the concurrency constraint at the mutex stub.**
+- [x] **0.9 — Write the concurrency constraint at the mutex stub.**
   Add a comment at `src/starkernel/vm/host/shim.c:415` stating that the no-op is correct
   only while nothing in interrupt context mutates shared structure, and that making it a
   real spinlock would deadlock a single hart.