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

build.rs

@@ -0,0 +1,56 @@
+//! Build script for XOmB
+//!
+//! Assembles the multiboot2 boot stub when building for bare metal target.
+
+use std::env;
+use std::path::PathBuf;
+use std::process::Command;
+
+fn main() {
+    let target = env::var("TARGET").unwrap_or_default();
+    let out_dir = PathBuf::from(env::var("OUT_DIR").unwrap());
+
+    // Only build assembly for multiboot2 target
+    if (target.contains("none") || target.contains("xomb")) && env::var("CARGO_FEATURE_MULTIBOOT2").is_ok() {
+        println!("cargo:rerun-if-changed=src/boot/multiboot2_header.asm");
+        println!("cargo:rerun-if-changed=linker-multiboot2.ld");
+
+        // Assemble with NASM (handles 32/64-bit mixing correctly)
+        let obj_path = out_dir.join("multiboot2_header.o");
+        let status = Command::new("nasm")
+            .args([
+                "-f", "elf64",
+                "-o", obj_path.to_str().unwrap(),
+                "src/boot/multiboot2_header.asm",
+            ])
+            .status()
+            .expect("Failed to run NASM. Install nasm package.");
+
+        if !status.success() {
+            panic!("Failed to assemble multiboot2_header.asm");
+        }
+
+        // Create static library
+        let lib_path = out_dir.join("libboot_asm.a");
+        let status = Command::new("ar")
+            .args([
+                "crus",
+                lib_path.to_str().unwrap(),
+                obj_path.to_str().unwrap(),
+            ])
+            .status()
+            .expect("Failed to run ar");
+
+        if !status.success() {
+            panic!("Failed to create libboot_asm.a");
+        }
+
+        // Link the assembly library
+        println!("cargo:rustc-link-search=native={}", out_dir.display());
+        println!("cargo:rustc-link-lib=static=boot_asm");
+
+        // Use custom linker script
+        println!("cargo:rustc-link-arg=-Tlinker-multiboot2.ld");
+        println!("cargo:rustc-link-arg=--gc-sections");
+    }
+}