partition: add MBR parser (Etapa 2)

partition: add MBR parser (Etapa 2)

- Added mbr_partition_entry_t structure for 16-byte partition entries
- Implemented partition_scan_mbr() to read and parse MBR sector
- Validates MBR signature (0xAA55) at offset 510
- Parses 4 primary partition entries starting at offset 446
- Skips empty partitions (type 0)
- Generates partition names (e.g. hda1, vda2)
- Silently skips devices without valid MBR (no errors)
- Tests: 124/124 PASS