Add devicefs VFS driver mounted at /dev

Implement device filesystem subsystem that provides a VFS interface at
/dev for exposing block and character devices. Drivers register devices
via devicefs_register_block() or devicefs_register_char(), and the
devicefs assigns sequential numbers per device class (e.g., hdd1, hdd2).

Features:
- Block device ops: read_sectors, write_sectors, sector_size, sector_count
- Character device ops: read, write
- VFS integration: readdir lists devices, finddir looks up by name
- Byte-offset to sector translation for block device reads/writes
- Auto-numbering: devices named classN where N starts at 1 per class

Also checks off 'ls' task in README.

src/kernel.c

@@ -17,6 +17,7 @@
 #include "cpio.h"
 #include "vfs.h"
 #include "initrd_fs.h"
+#include "devicefs.h"
 #include "keyboard.h"
 #include "framebuffer.h"
 
@@ -247,6 +248,10 @@ void kernel_main(uint32_t magic, uint32_t addr) {
         }
     }
 
+    init_devicefs();
+    EARLY_PRINT("DEV ");
+    offset_print("Devicefs initialized\n");
+
     init_tss();
     EARLY_PRINT("TSS ");
     offset_print("TSS initialized\n");