[Phase 2.1] GDT user space segments + heap growth

- Restructure GDT to add kernel_data/user_data/user_code in the order
  required for SYSCALL/SYSRET ABI:
    0x08 kernel code, 0x10 kernel data, 0x18 user data, 0x20 user code, 0x28 TSS
  STAR MSR values: STAR[47:32]=0x08, STAR[63:48]=0x10
- Add TSS.privilege_stack_table[0] (RSP0) with 8 KiB static initial stack
  for Ring3→Ring0 hardware interrupt transitions
- Expose GDT static and all Selectors fields as pub (needed by syscall module)
- Add set_kernel_stack(VirtAddr) for scheduler to update RSP0 per-process
- Grow HEAP_SIZE 100 KiB → 4 MiB to support process table + kernel stacks
- Fix pre-existing lifetime elision lint in allocator.rs
- Update flake.nix: add cpio, busybox, gdb, binutils, e2fsprogs
- Update NOTES.md with decisions and next steps

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 27 insertions, 8 deletions

diff --git a/flake.nix b/flake.nix
index ace0bf6..6a2215a 100644
--- a/flake.nix
+++ b/flake.nix
@@ -1,20 +1,39 @@
 {
-  description = "Dev shell for kernel/build environment";
+  description = "Dev shell for Strix OS kernel development";
 
   inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
 
-  outputs = { self, nixpkgs }: 
+  outputs = { self, nixpkgs }:
     let
       system = "x86_64-linux";
       pkgs = import nixpkgs { inherit system; };
     in {
       devShells.${system}.default = pkgs.mkShell {
-	packages = with pkgs; [
-    rustup # Rust
-    qemu # VM 
-	];
-	
+        packages = with pkgs; [
+          # Rust toolchain (nightly specified by rust-toolchain file)
+          rustup
+
+          # QEMU for running kernel and integration tests
+          qemu
+
+          # cpio: required by build.rs to pack the initramfs archive
+          cpio
+
+          # busybox statically linked: placed in initramfs/ for the embedded rescue shell
+          busybox
+
+          # Debugging and inspection tools
+          gdb
+          binutils   # objdump, nm, readelf
+
+          # Disk image tools for testing ext2/ext4 (Phase 4+)
+          e2fsprogs  # mkfs.ext2, mkfs.ext4, debugfs
+        ];
+
+        # Ensure cargo uses the nightly toolchain from rust-toolchain
+        shellHook = ''
+          rustup show
+        '';
       };
     };
 }
-