chore(devshell): add rustfmt + clippy + libclang for parity with CI (#32)

The `nix develop` shell brings the build inputs of the `visage` package
via `inputsFrom`, but not the cargo subcommands CI gates on:

  - `cargo fmt --all -- --check`
  - `cargo clippy --workspace -- -D warnings`

`dtolnay/rust-toolchain@stable` bundles both in the CI workflow; the
devshell didn't, so a contributor running `nix develop && cargo fmt`
hit "error: no such command: `fmt`" — same for clippy.

Also exposes `llvmPackages.libclang` + `LIBCLANG_PATH`, since
`v4l2-sys-mit` runs `bindgen` at build time and fails with
"Unable to find libclang" otherwise. Surfaced when running
`cargo check -p visaged` locally.

Verified: `cargo fmt --all -- --check` and `cargo clippy --workspace`
now run inside `nix develop` without further env tweaking.

No effect on CI (CI doesn't use the devshell). No effect on packaged
outputs (this only touches `devShells.default`).

Author: ccross2

flake.nix

@@ -26,10 +26,21 @@
         devShells.default = pkgs.mkShell {
           inputsFrom = [ visage ];
           packages = with pkgs; [
+            # Toolchain extras that match the CI gates
+            # (`cargo fmt --check`, `cargo clippy -- -D warnings`).
+            # `inputsFrom = [ visage ]` brings the compiler but not these.
+            rustfmt
+            clippy
+            # `bindgen` (transitively via `v4l2-sys-mit`) needs libclang.so
+            # at build time; without LIBCLANG_PATH set, `cargo build -p
+            # visaged` in the devshell fails with "Unable to find libclang".
+            llvmPackages.libclang
             rust-analyzer
             cargo-deb
             cargo-watch
           ];
+          # Tell bindgen where to find libclang at build time.
+          LIBCLANG_PATH = "${pkgs.llvmPackages.libclang.lib}/lib";
         };
       }
     ) // {