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>
