chore(devshell): add a Nix flake dev environment

flake.nix

@@ -0,0 +1,72 @@
+{
+  description = "TrUAPI development environment";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    rust-overlay = {
+      url = "github:oxalica/rust-overlay";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, rust-overlay, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs {
+          inherit system;
+          overlays = [ rust-overlay.overlays.default ];
+        };
+
+        stableToolchain = pkgs.rust-bin.stable.latest.default.override {
+          targets = [ "wasm32-unknown-unknown" ];
+        };
+
+        nightlyToolchain = pkgs.rust-bin.selectLatestNightlyWith
+          (toolchain: toolchain.default);
+      in
+      {
+        devShells.default = pkgs.mkShell {
+          packages = with pkgs; [
+            rustup
+            nodejs_22
+            yarn
+            bun
+            wasm-pack
+            wasm-bindgen-cli
+            binaryen
+            git
+            pkg-config
+            cargo-deny
+          ];
+
+          shellHook = ''
+            rustup_home="''${RUSTUP_HOME:-''$HOME/.rustup}"
+            host_triple="$(${stableToolchain}/bin/rustc --print host-tuple)"
+            toolchains_dir="$rustup_home/toolchains"
+            mkdir -p "$toolchains_dir"
+
+            # rustup >= 1.28 rejects channel-like names for `rustup toolchain
+            # link`, so link the pinned rust-overlay toolchains directly under
+            # the official channel directory names. `cargo +stable` and
+            # `cargo +nightly` then dispatch to the pinned store paths instead
+            # of a network download.
+            #
+            # Note: toolchains linked to read-only store paths cannot carry a
+            # dist manifest, so manifest-dependent introspection (`rustup
+            # show`, `rustup target list --toolchain <channel>`) errors with
+            # "Missing manifest in toolchain". Use `rustup toolchain list`,
+            # `rustup show active-toolchain`, and `rustc +<channel> --version`
+            # instead; compilation and `+<channel>` dispatch are unaffected.
+            for name in stable nightly; do
+              if [ "$name" = stable ]; then src="${stableToolchain}"; else src="${nightlyToolchain}"; fi
+              dir="$toolchains_dir/$name-$host_triple"
+              [ -L "$dir" ] || rm -rf -- "$dir"
+              ln -sfn "$src" "$dir"
+            done
+
+            rustup default stable
+          '';
+        };
+      });
+}