feat: add lab-rv32i-freertos-semaphore-notify card

README.md

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+# K12 — semaphores and task notifications
+
+K12 separates three synchronization meanings behind small C++ wrappers:
+
+- `BinarySemaphore` is an empty/full signal with no payload or owner;
+- `CountingSemaphore<Max>` models a bounded count of available resources;
+- `TaskNotification` is a non-owning view of one task's TCB notification slot,
+  with explicit action/value semantics.
+
+Both semaphore wrappers use caller-owned static control blocks. The
+notification wrapper allocates no kernel object: its state is already inside
+the target task's TCB.
+
+## Hazard3 evidence
+
+```text
+binary count                0 -> 1 -> 0
+second binary give          false (already full)
+counting slots              2 -> 1 -> 0
+third counting take         waiter Blocked
+one give                    wakes waiter; count consumed back to 0
+notification take result    1
+explicit overwrite value    0xA5A55A5A
+```
+
+The same high-priority waiter is observed in `eBlocked` for counting take,
+notification take and value wait. `vTaskSuspendAll` creates a stable checkpoint
+after notification give but before the waiter consumes the TCB value.
+
+## Build
+
+```sh
+make check
+```
+
+## Debug
+
+```gdb
+b synchronization_debug_checkpoint
+p g_binary_initial_count
+p g_binary_after_give
+p g_binary_second_give
+p g_counting_initial_count
+p g_count_after_take1
+p g_count_after_take2
+p g_waiter_state_for_counting
+p g_waiter_state_for_notification
+p g_notification_pending_checkpoint
+p g_notification_take_value
+p/x g_notification_message_value
+p g_sync_pass
+```
+
+## Scope boundary
+
+A semaphore is not payload storage. A notification has exactly one receiving
+task per TCB slot and action/clear policy must be explicit; it is not a
+multi-consumer queue. Binary/counting `take` and `notify wait` use different
+state machines even when their visible wakeup looks identical.