Add Nix flake: crane + fenix build, treefmt-nix formatting

Uses flake-parts to organize outputs, with nix/rust.nix defining a
single craneLib/toolchain/commonArgs environment shared by the package
build (nix/rust.nix) and the dev shell (nix/devshell.nix), so both stay
close to identical. Deps are cached separately from the crate itself via
crane's buildDepsOnly, giving incremental rebuilds. Checks cover clippy,
rustfmt, cargo-doc, cargo-audit, and nextest; treefmt-nix wires up
`nix fmt` for Rust/Nix/TOML files.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018ALEXkc7pTro7tF1WcUuKb

nix/treefmt.nix

@@ -0,0 +1,14 @@
+# treefmt-nix module: wires up `nix fmt` and the `formatting` check.
+_: {
+  perSystem = _: {
+    treefmt = {
+      projectRootFile = "flake.nix";
+
+      programs.rustfmt.enable = true;
+      programs.nixfmt.enable = true;
+      programs.deadnix.enable = true;
+      programs.statix.enable = true;
+      programs.taplo.enable = true;
+    };
+  };
+}