feat: /dev/hdX block device nodes and /proc/dmesg

feat: /dev/hdX block device nodes and /proc/dmesg

1. ATA block devices in devfs:
   Detected ATA drives (hda, hdb, hdc, hdd) are now registered as
   block device nodes in /dev via ata_register_devfs(). Each node
   supports read/write at byte offsets (with sector-aligned I/O
   internally). Previously, drives were detected but invisible
   in /dev, so 'ls /dev' showed no disk devices and mount had to
   use ata_name_to_drive() internally.

2. /proc/dmesg:
   Added /proc/dmesg to procfs that reads the kprintf ring buffer
   via klog_read(). Uses heap allocation (16KB) to avoid kernel
   stack overflow. The 'dmesg' command now works correctly.