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

[ccross2]

Summary

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

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

dtolnay/rust-toolchain@stable bundles both in the CI workflow (.github/workflows/ci.yml); our devShells.default doesn't. A contributor running nix develop && cargo fmt hits 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. Caught when running cargo check -p visaged in the devshell.

Diff

devShells.default = pkgs.mkShell {
  inputsFrom = [ visage ];
  packages = with pkgs; [
+    rustfmt
+    clippy
+    llvmPackages.libclang
     rust-analyzer
     cargo-deb
     cargo-watch
   ];
+  LIBCLANG_PATH = "${pkgs.llvmPackages.libclang.lib}/lib";
};

Scope

• No effect on CI (CI doesn't use the devshell).
• No effect on packaged outputs (only touches devShells.default).
• No version bump.

Verification

In a freshly-entered nix develop:

$ which rustfmt cargo-clippy
/nix/store/...-rustfmt-1.95.0/bin/rustfmt
/nix/store/...-clippy-1.95.0/bin/cargo-clippy

$ echo $LIBCLANG_PATH
/nix/store/...-clang-21.1.8-lib/lib

$ cargo fmt --all -- --check    # clean on current main
$ cargo clippy --workspace -- -D warnings    # CI parity

Catches a real contributor friction point — surfaced by my own attempts to verify the SIGTERM fix (#30) and the PAM sweep (#31) before pushing.

🤖 Generated with Claude Code