Add ENOTSUP fallback in mutex lock/unlock for pre-RTOS operation

[Copilot]

Dmod_Mutex_Lock returns -ENOTSUP when the RTOS scheduler hasn't started yet, causing all DMVFS operations (including filesystem mounting) to fail with "Failed to lock DMVFS mutex".

Changes

• src/dmvfs.c
  • Add #include <errno.h>
  • lock_mutex(): on -ENOTSUP from Dmod_Mutex_Lock, fall back to Dmod_EnterCritical() instead of returning failure
  • unlock_mutex(): on -ENOTSUP from Dmod_Mutex_Unlock, call Dmod_ExitCritical() to match the lock-side fallback

// Before: any non-zero return from Dmod_Mutex_Lock was a hard failure
return (Dmod_Mutex_Lock(g_mutex) == 0);

// After: ENOTSUP means RTOS not started — transparently use critical sections
int ret = Dmod_Mutex_Lock(g_mutex);
if (ret == 0)        { return true; }
if (ret == -ENOTSUP) { Dmod_EnterCritical(); return true; }
return false;

The same symmetry is applied in unlock_mutex — if Dmod_Mutex_Unlock returns -ENOTSUP, Dmod_ExitCritical() is called to pair with the critical section entered during lock.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Dodać fallback mutexów dla ENOTSUP</issue_title>
<issue_description>Ostatnio do mutexów zostało dodane sprawdzanie czy system już wystartował. Niestety przez to, jeśli próbujemy zamontować jakiś system plików przed startem RTOSa, to mutexy zwracają errno -ENOTSUP, co powoduje problem z montowaniem systemów plików:

[INFO] To debug module 'dmell' in gdb: add-symbol-file <MODULE_ELF_FILE> 0x200186f8
[ERROR] Failed to lock DMVFS mutex
[INFO] Config filesystem found in ROM: addr=0x8046B3B, size=774 bytes
[ERROR] Failed to lock DMVFS mutex
[ERROR] Failed to mount config filesystem at /configs/
[ERROR] Failed to lock DMVFS mutex

Trzeba dodać sprawdzenie, czy to jest taki kod błędu i w takim przypadku założyć, że RTOS nie wystartował i że sekcje krytyczne wystarczą</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Dodać fallback mutexów dla ENOTSUP #51

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