Add MBR partition driver

- Scan sector 0 of all hdd devices for MBR signature (0xAA55)
- Parse 4 partition entries, register non-empty ones as sub-devices
- Sub-devices named hddNmbrY (e.g., hdd1mbr1) via devicefs
- LBA translation: sub-device reads/writes offset by partition start LBA
- Partition type constants for FAT12/16/32, Linux, NTFS, etc.
- IDE improvements: floating bus detection (0xFF), channel presence check
- Tested: detects FAT32 LBA partition (type=0x0C) on test disk
- Check off devicefs, IDE, MBR in README

src/CMakeLists.txt

@@ -21,6 +21,7 @@ add_executable(kernel
     initrd_fs.c
     devicefs.c
     ide.c
+    mbr.c
     env.c
     keyboard.c
     interrupts.S