✨♻️ ci, nix, rust: Set up CI and switch to rust-overlay toolchain

- Added `.github/workflows/test.yaml` to run `cargo test` on push and pull_request events.
- Replaced `fenix` with `rust-overlay` in flake inputs for Rust toolchain management.
- Added `rust-toolchain.toml` specifying stable Rust with required components and targets.
- Updated `flake.nix` to use `rust-bin.fromRustupToolchainFile` and removed `RUST_SRC_PATH`.

flake.lock

@@ -15,25 +15,6 @@
         "type": "github"
       }
     },
-    "fenix": {
-      "inputs": {
-        "nixpkgs": "nixpkgs",
-        "rust-analyzer-src": "rust-analyzer-src"
-      },
-      "locked": {
-        "lastModified": 1754116698,
-        "narHash": "sha256-2cMDogXcqv975o191g0/6ZM0UolJ2X0ChuuLaNKbNAM=",
-        "owner": "nix-community",
-        "repo": "fenix",
-        "rev": "acdb7d8af407ea8aee21b028dc58b4bc51a7849a",
-        "type": "github"
-      },
-      "original": {
-        "owner": "nix-community",
-        "repo": "fenix",
-        "type": "github"
-      }
-    },
     "flake-parts": {
       "inputs": {
         "nixpkgs-lib": "nixpkgs-lib"
@@ -54,16 +35,16 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1753939845,
-        "narHash": "sha256-K2ViRJfdVGE8tpJejs8Qpvvejks1+A4GQej/lBk5y7I=",
-        "owner": "nixos",
+        "lastModified": 1754060105,
+        "narHash": "sha256-di5L6e5Iiv+oegS07j9h23FdqEpXn0ZQqMlDOEMw1EY=",
+        "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "94def634a20494ee057c76998843c015909d6311",
+        "rev": "e7eabdc701d7dbb810fd91a97ec358caa4c1fc50",
         "type": "github"
       },
       "original": {
-        "owner": "nixos",
-        "ref": "nixos-unstable",
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
         "repo": "nixpkgs",
         "type": "github"
       }
@@ -85,11 +66,11 @@
     },
     "nixpkgs_2": {
       "locked": {
-        "lastModified": 1754060105,
-        "narHash": "sha256-di5L6e5Iiv+oegS07j9h23FdqEpXn0ZQqMlDOEMw1EY=",
+        "lastModified": 1744536153,
+        "narHash": "sha256-awS2zRgF4uTwrOKwwiJcByDzDOdo3Q1rPZbiHQg/N38=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "e7eabdc701d7dbb810fd91a97ec358caa4c1fc50",
+        "rev": "18dd725c29603f582cf1900e0d25f9f1063dbf11",
         "type": "github"
       },
       "original": {
@@ -102,25 +83,26 @@
     "root": {
       "inputs": {
         "crane": "crane",
-        "fenix": "fenix",
         "flake-parts": "flake-parts",
-        "nixpkgs": "nixpkgs_2"
+        "nixpkgs": "nixpkgs",
+        "rust-overlay": "rust-overlay"
       }
     },
-    "rust-analyzer-src": {
-      "flake": false,
+    "rust-overlay": {
+      "inputs": {
+        "nixpkgs": "nixpkgs_2"
+      },
       "locked": {
-        "lastModified": 1753974682,
-        "narHash": "sha256-Ya4Ecj8JaKkapgYCCybzZBcypXpblhuG0hVDzdy2zyo=",
-        "owner": "rust-lang",
-        "repo": "rust-analyzer",
-        "rev": "68e7ec90bf29c9b17d0dcdb3358de5dee7f4afb5",
+        "lastModified": 1754966322,
+        "narHash": "sha256-7f/LH60DnjjQVKbXAsHIniGaU7ixVM7eWU3hyjT24YI=",
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "rev": "7c13cec2e3828d964b9980d0ffd680bd8d4dce90",
         "type": "github"
       },
       "original": {
-        "owner": "rust-lang",
-        "ref": "nightly",
-        "repo": "rust-analyzer",
+        "owner": "oxalica",
+        "repo": "rust-overlay",
         "type": "github"
       }
     }