Fix mount point visibility in directory listings

[Copilot]

Directory listings (opendir/readdir) only returned entries from the underlying filesystem, omitting subdirectories that are mount points. Mounting ramfs at / and dmffs at /configs would list files in / but not show the configs directory.

Changes

Core Implementation

• Added dir_handle_t wrapper structure to track directory state and mount point injection
• Modified dmvfs_readdir to inject mount points as DMFSI_ATTR_DIRECTORY entries after filesystem entries exhausted
• Modified dmvfs_opendir/dmvfs_closedir to create/cleanup wrappers

Helper Functions

• is_direct_child_mount(dir_path, mount_path): Returns true if mount point is exactly one level below directory
• get_basename(path, buffer, size): Extracts final path component for mount point names

Bug Fixes

• Fixed double-free of abs_path in dmvfs_opendir
• Fixed DMVFS_VERSION format string compilation error

Example

dmvfs_mount_fs("ramfs", "/", NULL);
dmvfs_mount_fs("dmffs", "/configs", NULL);

void* dp;
dmvfs_opendir(&dp, "/");

dmfsi_dir_entry_t entry;
while (dmvfs_readdir(dp, &entry) == 0) {
    printf("%s\n", entry.name);
    // Now includes "configs" directory entry
}

Mount points appear as directories with DMFSI_ATTR_DIRECTORY set. The wrapper is transparent to callers; existing code works unchanged.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Ukryte pliki z listowania</issue_title>
<issue_description>Jeśli zrobimy tak:

• W ścieżce / zamontujemy system plików np ramfs
• W ścieżce /configs zamontujemy inny system plików np dmffs
• I następnie zawołamy opendir na / to dostaniemy katalogi tylko z ramfs, a nie zobaczymy zamontowanego /configs - powinniśmy również go otrzymać</issue_description>

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

• Fixes Ukryte pliki z listowania #49

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