feat: add nix flake with fenix rust toolchain

Configure hermetic development environment:

- fenix toolchain from rust-toolchain.toml
- clangStdenv for Tree-sitter C grammar compilation
- Multi-platform devShells (x86_64/aarch64, linux/darwin)
- Generated flake.lock with pinned inputs

flake.nix

@@ -0,0 +1,52 @@
+{
+  description = "nrd.sh - bespoke static site compiler";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    fenix = {
+      url = "github:nix-community/fenix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs =
+    {
+      self,
+      nixpkgs,
+      fenix,
+    }:
+    let
+      systems = [
+        "x86_64-linux"
+        "aarch64-linux"
+        "x86_64-darwin"
+        "aarch64-darwin"
+      ];
+      forAllSystems = fn: nixpkgs.lib.genAttrs systems (system: fn system);
+    in
+    {
+      devShells = forAllSystems (
+        system:
+        let
+          pkgs = nixpkgs.legacyPackages.${system};
+          fenixPkgs = fenix.packages.${system};
+          toolchain = fenixPkgs.fromToolchainFile { file = ./rust-toolchain.toml; sha256 = "sha256-vra6TkHITpwRyA5oBKAHSX0Mi6CBDNQD+ryPSpxFsfg=";};
+        in
+        {
+          default = pkgs.mkShell.override { stdenv = pkgs.clangStdenv; } {
+            RUST_SRC_PATH = "${toolchain}/lib/rustlib/src/rust/library";
+            packages =
+              [
+                toolchain
+                fenixPkgs.default.rustfmt
+                pkgs.pkg-config
+              ]
+              ++ pkgs.lib.optionals pkgs.stdenv.isDarwin [
+                pkgs.apple-sdk
+                pkgs.libiconv
+              ];
+          };
+        }
+      );
+    };
+}