chore(deps): update dev dependencies and toolchain

Update development dependencies to include:
- llvm-tools-preview for better LLVM tooling support.
- cargo-llvm-cov for improved code coverage reporting.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/gemma-4-26B-A4B-it-GGUF:UD-Q5_K_XL)

flake.nix

@@ -24,7 +24,7 @@
       };
 
       rustToolchain = pkgs.rust-bin.stable.latest.default.override {
-        extensions = ["rust-src" "clippy" "rustfmt"];
+        extensions = ["rust-src" "clippy" "rustfmt" "llvm-tools-preview"];
       };
 
       generatedCargoNix = crate2nix.tools.${system}.generatedCargoNix {
@@ -43,6 +43,7 @@
           pkgs.rust-analyzer
           crate2nix.packages.${system}.default
           pkgs.cargo-expand
+          pkgs.cargo-llvm-cov
         ];
         RUST_BACKTRACE = "1";
       };