feat(build): built-in kernel module support

[yrakcaz]

Adds support for compiling kernel modules directly into the kernel binary, making them available at boot before the filesystem is mounted.

Introduces xtask/, a cargo-xtask build orchestrator with a single build-kernel subcommand that runs two cargo build passes inside kernel/:

• First pass produces libkernel.rlib
• Second pass detects the rlib, compiles configured modules against it, and embeds them as aligned byte arrays via include_bytes!

The double-pass is necessary because build.rs runs before libkernel.rlib exists on the first build. cargo:rerun-if-changed=libkernel.rlib triggers the second pass automatically. When no built-in modules are configured the second pass is a fast no-op.

• build/module.rs: new build script module that compiles each .so listed in [modules] builtin and generates builtin_modules.rs with #[repr(align(8))]-wrapped include_bytes! statics
• src/module/mod.rs: load_builtin_modules() iterates BUILTIN_MODULES and calls Module::load() on each embedded blob
• src/kernel.rs: calls load_builtin_modules() at boot
• default.build-config.toml: new [modules] section with builtin = []
• kernel/README.md: documents the built-in modules workflow

New builtin_module job (matrix: x86 × x86_64, debug × release):

• configures build-config.toml with builtin = ["ps2", "e1000"]
• builds with cargo xtask build-kernel
• verifies builtin_modules.rs references libps2.so and libe1000.so