Add DEVFS (device filesystem) and integrate FS_DEV into VFS and tools

[pradosh-arduino]

Motivation

• Provide a virtual /dev filesystem exposing character-like nodes (null, zero, random, urandom) and block-device nodes for direct access to block devices.
• Integrate device nodes into existing VFS, shell, syscalls, and tooling so users can mount /dev, list devices, and perform read/write operations via standard VFS calls.

Description

• Added FS_DEV to partition_fs_type_t in ahci.h to represent the device filesystem.
• Implemented devfs layer with new header source/includes/filesystems/layers/dev.h and implementation source/kernel/C/filesystems/layers/dev.c providing devfs_init, devfs_open, devfs_read, devfs_write, devfs_close, and devfs_ls and support for both pseudo devices and block-device passthrough.
• Integrated devfs into the VFS (vfs.c) by handling FS_DEV in vfs_open, vfs_read, vfs_write, vfs_close, vfs_ls, and vfs_cd paths and by exposing file position behavior in stream.c (fd_file_size, fd_pos_ptr).
• Extended syscall helpers in syscalls.c to handle FS_DEV in stat resolution and directory enumeration (getdents64) to list pseudo devices and block devices.
• Added support for mounting/unmounting dev in shell commands: mount.c now accepts dev and calls devfs_init, umount.c handles FS_DEV cleanup, and lsblk.c prints dev filesystem info and marks it read-only appropriately.
• Included necessary header imports across modified files for proc.h and dev.h.

Testing

• Performed a full build with make which completed successfully.
• Ran the repository's filesystem-related automated tests (make test / VFS smoke tests) including vfs open/read/write/ls scenarios for /dev entries which completed without failures.
• Executed automated getdents/directory enumeration tests that verified /dev returns pseudo nodes and block-device entries, and these tests passed.

---

Codex Task