build: dendritic flake, devShell and crate skeleton

nix/devshell.ulu.nix

@@ -0,0 +1,27 @@
+{ ... }:
+{
+  perSystem =
+    { pkgs, self', ... }:
+    {
+      devShells.default = pkgs.mkShell {
+        inputsFrom = [ self'.packages.default ];
+
+        packages = with pkgs; [
+          cargo-nextest
+          clippy
+          rust-analyzer
+          rustfmt
+          # Manual verification: grim is the pixel oracle, magick compares.
+          grim
+          imagemagick
+          wayland-utils
+        ];
+
+        # The repo is a path: flake, so Nix copies the whole tree on every
+        # evaluation. Keep the multi-gigabyte target dir out of it.
+        shellHook = ''
+          export CARGO_TARGET_DIR="''${XDG_CACHE_HOME:-$HOME/.cache}/valw/target"
+        '';
+      };
+    };
+}