Restore semaphore value on timed wait failure

[Old-Ding]

Summary

• restore the POSIX semaphore count when sem_timedwait() fails to take the FreeRTOS semaphore
• keep sem_post() and the existing wait model unchanged

Root cause

sem_timedwait() decrements pxSem->value before attempting to wait. If the wait times out, the semaphore is not acquired, but the decrement was not undone. Repeated timed waits can therefore drive the stored semaphore value increasingly negative.

Closes #26

Testing

• git diff --check
• verified the diff is limited to FreeRTOS-Plus-POSIX/source/FreeRTOS_POSIX_semaphore.c
• verified the file keeps CRLF line endings

Not run: cmake -S test/posix_build_test -B test/posix_build_test/build and build. This machine does not have cmake, gcc, clang, make, or ninja installed.

[Old-Ding]

Follow-up verification:

• Rechecked the sem_timedwait() path: pxSem->value is decremented before the wait attempt, and this PR restores it only when xSemaphoreTake() fails. Successful waits keep the existing consume semantics.
• git diff --check origin/main...HEAD passes on the branch.
• I found no duplicate open PR for this issue.

I still do not have CMake or a C toolchain available on this Windows host to run test/posix_build_test locally.