kernel: Fix critical bugs and implement FAT filesystem

This commit addresses several critical architectural issues in Bengal
Tiger OS and implements missing core functionality:

1.  **Memory Management**: Fixed a conflict where the PMM could allocate
    pages used by the page directory/tables. Expanded identity mapping
    to 16MB to cover the kernel and the initial heap (at 4MB),
    preventing page faults during initialization.
2.  **Heap Allocator**: Re-implemented `kmalloc_aligned` to store the
    original pointer, allowing aligned allocations to be safely freed
    via `kfree`.
3.  **FAT Filesystem**: Replaced the stub FAT driver with a functional
    FAT12/16 implementation that supports boot sector parsing,
    directory traversal, and cluster chain following.
4.  **Shell**: Fixed backspace logic and updated `ls` to use the real
    filesystem driver.
5.  **Build System**: Updated the Makefile to handle missing ISO creation
    tools gracefully and added a `.gitignore` to exclude build artifacts.

These changes make the OS bootable and functional for real-world
use cases involving file access and stable memory allocation.

Co-authored-by: mahmud-r-farhan <114731414+mahmud-r-farhan@users.noreply.github.com>

Author: google-labs-jules[bot]

.gitignore

@@ -0,0 +1,5 @@
+build/
+iso/
+bengaltiger.iso
+*.o
+*.bin