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/kernel.c

@@ -18,6 +18,7 @@
 #include "vfs.h"
 #include "initrd_fs.h"
 #include "devicefs.h"
+#include "mbr.h"
 #include "keyboard.h"
 #include "framebuffer.h"
 
@@ -286,6 +287,10 @@ void kernel_main(uint32_t magic, uint32_t addr) {
     EARLY_PRINT("DRV ");
     offset_print("Drivers initialized\n");
 
+    /* Scan for MBR partitions on detected hard drives */
+    init_mbr();
+    offset_print("MBR scan complete\n");
+
     /* At this point the VGA driver has been initialized and taken over
      * the display. The early VGA text is no longer visible. */