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.lock

@@ -0,0 +1,66 @@
+{
+  "nodes": {
+    "fenix": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ],
+        "rust-analyzer-src": "rust-analyzer-src"
+      },
+      "locked": {
+        "lastModified": 1769237067,
+        "narHash": "sha256-o2UfbFRcL/UClXaDI8NR3WAulOoda55rdw0oVXAst0A=",
+        "owner": "nix-community",
+        "repo": "fenix",
+        "rev": "93523fa073f781d3d02d326cdbb85f8709b00c40",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-community",
+        "repo": "fenix",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1769268028,
+        "narHash": "sha256-mAdJpV0e5IGZjnE4f/8uf0E4hQR7ptRP00gnZKUOdMo=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "ab9fbbcf4858bd6d40ba2bbec37ceb4ab6e1f562",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "fenix": "fenix",
+        "nixpkgs": "nixpkgs"
+      }
+    },
+    "rust-analyzer-src": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1769166439,
+        "narHash": "sha256-HjSoNRYe8tPdzfhJ0IBQrrJATOrtVKOHvk8OoTew1Do=",
+        "owner": "rust-lang",
+        "repo": "rust-analyzer",
+        "rev": "39018acf3477a26f4e6d2ead6a48954d623adcaa",
+        "type": "github"
+      },
+      "original": {
+        "owner": "rust-lang",
+        "ref": "nightly",
+        "repo": "rust-analyzer",
+        "type": "github"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}