Fix subdirectory listing under /dev mount point

[Copilot]

ls /dev/dmgpio8 failed with "Directory not found" because read_driver_parent_directory() built paths without a leading / (e.g. "dmgpio8/") while DMVFS strips the mount prefix and passes paths with a leading / (e.g. "/dmgpio8"), causing all comparisons in is_directory() to fail.

Changes

• read_driver_parent_directory — prepend / to non-root parent paths:

  // Before
  Dmod_SnPrintf(path_buffer, buffer_size, "%s%u/", driver_name, node->dev_num.major);
  // After
  Dmod_SnPrintf(path_buffer, buffer_size, "/%s%u/", driver_name, node->dev_num.major);

• read_next_subdir_name — when listing from root /, skip the leading / now present in full_path before extracting the next component:

  // Before: assumed full_path had no leading slash
  start = full_path;
  // After
  start = (full_path[0] == '/') ? full_path + 1 : full_path;

Original prompt

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This section details on the original issue you should resolve

<issue_title>Problem z listowaniem katalogów</issue_title>
<issue_description>Już jest lepiej, bo da się wylistować główny katalog, ale już plików wewnątrz nie:

stm32f746xg@/> ls /dev
dmgpio8  
[WARN] dmheap: Module ls is not registered.
stm32f746xg@/> ls /dev/dmgpio8
dmdevfs: [ERROR] Directory not found: /dmgpio8
dmdevfs: [ERROR] File not found: /dmgpio8
[ERROR] Failed to open file '/dev/dmgpio8'
ls: [ERROR] Cannot access '/dev/dmgpio8': No such file or directory
[WARN] dmheap: Module ls is not registered.
stm32f746xg@/> 

```</issue_description>

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

<comments>
</comments>

• Fixes Problem z listowaniem katalogów #36

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