Implement missing DMFSI interface functions for device driver file operations

[Copilot]

The dmdevfs module had 14 unimplemented DMFSI interface functions marked with TODOs. This PR completes the implementation to enable file-like operations on hardware device drivers through the dmdrvi interface.

Implementation

File I/O Operations

• _fopen: Maps to dmdrvi_open() (context + flags → device handle)
• _fclose: Maps to dmdrvi_close() with proper cleanup of duplicated path string
• _fread/_fwrite: Call dmdrvi_read()/dmdrvi_write() which return size_t bytes transferred
• _getc/_putc: Thin wrappers around read/write for single byte operations

Position & Status Operations

• _lseek/_tell: Return error (character devices are non-seekable)
• _eof: Returns 0 (streaming devices have no EOF concept)
• _size: Queries dmdrvi_stat() if available

Buffer & Management Operations

• _fflush/_sync: Call dmdrvi_flush() when available
• _unlink/_rename: Return error (device nodes are immutable)

Key Design Decisions

Added file_handle_t structure to track:

typedef struct {
    driver_node_t* driver;      // Associated driver
    void* driver_handle;        // Device handle from dmdrvi_open
    const char* path;           // Duplicated path string
    int mode;                   // Open mode flags
    int attr;                   // File attributes
} file_handle_t;

Device drivers expose character device semantics (stream-oriented read/write) rather than file semantics (seekable, truncatable). Operations incompatible with this model return appropriate errors instead of failing silently.

Memory management uses DMOD allocation functions (Dmod_Malloc, Dmod_StrDup, Dmod_Free) consistently throughout.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Dodać implementację brakujących funkcji interfejsowych</issue_title>
<issue_description>Większość funkcji już została zaimplementowana, trzeba teraz dodać te brakujące</issue_description>

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

• Fixes Dodać implementację brakujących funkcji interfejsowych #8

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