Initial commit: XOmB exokernel foundation

Core kernel infrastructure:
- Multiboot2 boot with GRUB, long mode setup, higher-half kernel
- Serial port output for debugging
- Unified boot info abstraction for future UEFI support

Memory management:
- Physical frame allocator with bitmap tracking
- Page table manipulation via recursive mapping (PML4[510])
- Support for 4KB, 2MB, and 1GB page mappings
- TLB invalidation and proper NXE support

Build system:
- Cargo-based build with custom x86_64 target
- Makefile for QEMU and Bochs testing
- GRUB ISO generation for multiboot2 boot

boot/grub/grub.cfg

@@ -0,0 +1,11 @@
+# GRUB configuration for XOmB exokernel
+#
+# This file is embedded in the bootable disk image for Bochs.
+
+set timeout=0
+set default=0
+
+menuentry "XOmB Exokernel" {
+    multiboot2 /boot/xomb-multiboot2
+    boot
+}