feat: add lab-rv32i-freertos-mutex card

README.md

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+# K11 — Mutex, LockGuard and priority inheritance
+
+K11 wraps a caller-owned static FreeRTOS mutex in `Mutex` and pairs
+`lock(timeout)`/`unlock()` with a non-copyable `LockGuard<Mutex>`. The target
+experiment forces a deterministic inversion scenario:
+
+1. LOW (priority 1) takes the mutex;
+2. HIGH (priority 3) attempts to take it and blocks;
+3. MEDIUM (priority 2) is ready;
+4. LOW inherits priority 3, runs before MEDIUM and releases through the guard;
+5. HIGH acquires; LOW later returns to priority 1.
+
+## Hazard3 evidence
+
+```text
+LOW base priority             = 1
+LOW inherited priority        = 3
+LOW after release             = 1
+MEDIUM ran before release     = 0
+protected shared counter      = 11
+```
+
+The observed holder equals LOW/HIGH native handles at the appropriate points,
+HIGH is `eBlocked` while LOW holds the mutex, and the event order is
+LOW-take, HIGH-attempt, LOW-release, HIGH-take, MEDIUM-run.
+
+## Build
+
+```sh
+make check
+```
+
+Host tests prove destruction balances three early returns and that a second
+take of an ordinary non-recursive mutex fails. ABI checks keep static control in
+`.bss` and reject vtables, RTTI, exceptions and hosted runtime.
+
+## Debug
+
+```gdb
+b mutex_debug_checkpoint
+p g_low_base_priority
+p g_low_inherited_priority
+p g_low_priority_after_release
+p g_high_state_seen_by_low
+p g_medium_ran_before_release
+p g_owner_low
+p g_owner_high
+p g_sequence_low_take
+p g_sequence_high_attempt
+p g_sequence_low_release
+p g_sequence_high_take
+p g_sequence_medium_run
+p g_mutex_pass
+```
+
+## Scope boundary
+
+An ordinary mutex is task-context only, must be released by its owner and is
+not recursive. A binary semaphore does not provide mutex ownership or priority
+inheritance. `LockGuard` balances control flow but cannot make ISR use,
+non-owner unlock or unbounded critical work valid.