Add FAT32 write support

[pbalduino]

Context

Currently FAT32 filesystem is read-only. Need write support for file creation, modification, and save games.

Requirements

• File creation and deletion
• Directory creation
• File modification (append, truncate)
• FAT table updates
• Directory entry management

Dependencies

Related to #60 (file I/O syscalls)

Acceptance Criteria

• Can create files from userland
• Can write to existing files
• Can create directories
• Save game functionality works
• Tests for write operations