Add semaphore primitives to SAL RTOS interface

[Copilot]

SAL exposed mutex primitives but had no semaphore support, which left synchronization incomplete for RTOS-style usage. This change adds semaphore APIs to the SAL contract and default system-side implementation.

• SAL API surface (RTOS section)

  • Added new built-in SAL functions in inc/dmod_sal.h:
    • Dmod_Semaphore_New(uint32_t InitialValue)
    • Dmod_Semaphore_Wait(void* Semaphore)
    • Dmod_Semaphore_Post(void* Semaphore)
    • Dmod_Semaphore_Delete(void* Semaphore)

• Default system implementation (weak symbols)

  • Implemented semaphore primitives in src/system/if/dmod_if_rtos.c using POSIX semaphores (sem_t).
  • Matched existing SAL conventions for:
    • allocation/free lifecycle
    • null-handle validation
    • error-path logging
    • fallback behavior when pthread support is disabled

• Public system tests

  • Added tests/system/public/tests_dmod_semaphore.cpp and registered it in tests/system/public/CMakeLists.txt.
  • Covers nominal semaphore lifecycle and null-handle behavior.

• Integration docs

  • Extended SAL recommendation table in README.md to include semaphore primitives alongside mutexes.

void* sem = Dmod_Semaphore_New(1);
Dmod_Semaphore_Wait(sem);
/* critical section */
Dmod_Semaphore_Post(sem);
Dmod_Semaphore_Delete(sem);