build: dendritic flake, devShell and crate skeleton

.envrc

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+watch_file nix/*.ulu.nix
+use flake

.gitignore

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+/*
+!/.gitignore
+!/.envrc
+!/flake.nix
+!/flake.lock
+!/Cargo.toml
+!/Cargo.lock
+!/nix/
+!/src/
+!/docs/
+!/README.md

Cargo.toml

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+[package]
+name = "valw"
+version = "0.1.0"
+edition = "2024"
+description = "macOS-style screenshots for niri"
+license = "MIT"
+
+[dependencies]
+anyhow = "1.0.104"
+chrono = "0.4.45"
+clap = { version = "4.6.7", features = ["derive"] }
+image = { version = "0.25.10", default-features = false, features = ["png"] }
+libc = "0.2.189"
+niri-ipc = "=26.4.0"
+rustix = { version = "1.1.5", features = ["event", "fs", "pipe", "process", "stdio"] }
+serde = { version = "1.0.229", features = ["derive"] }
+serde_ignored = "0.1.14"
+smithay-client-toolkit = "0.21.1"
+toml = "1.1.6"
+tracing = "0.1.44"
+tracing-subscriber = { version = "0.3.23", features = ["env-filter"] }
+wayland-client = "0.31.15"
+wayland-protocols = { version = "0.32.13", features = ["client", "staging"] }
+wayland-protocols-wlr = { version = "0.3.12", features = ["client"] }
+wl-clipboard-rs = "0.9.4"
+
+[dev-dependencies]
+tempfile = "3.27.0"
+
+# Image encoding is painfully slow unoptimised; keep dev builds usable.
+[profile.dev.package."*"]
+opt-level = 3

flake.lock

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+{
+  "nodes": {
+    "flake-parts": {
+      "inputs": {
+        "nixpkgs-lib": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1788450739,
+        "narHash": "sha256-glZLQlzIn1fXH6PazR2iUmTo7kzzyYSshrWhLS9TqCU=",
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "rev": "31729ca8cbdb4fa927b34e5f4353e6a83f39e993",
+        "type": "github"
+      },
+      "original": {
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1790578696,
+        "narHash": "sha256-ZoxIApko70jCdbH3l20HWXOBaT2HZd87orzd2yJ9dVE=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "7a0f122f5090cf4c2ade2a13a0e229d4e19ba71f",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-parts": "flake-parts",
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

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+{
+  description = "valw, macOS-style screenshots for niri";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+
+    flake-parts = {
+      url = "github:hercules-ci/flake-parts";
+      inputs.nixpkgs-lib.follows = "nixpkgs";
+    };
+  };
+
+  outputs =
+    inputs@{ flake-parts, ... }:
+    flake-parts.lib.mkFlake { inherit inputs; } (
+      { lib, ... }:
+      let
+        inherit (lib.filesystem) listFilesRecursive;
+        inherit (lib.lists) filter;
+        inherit (lib.strings) hasSuffix;
+      in
+      {
+        systems = [
+          "x86_64-linux"
+          "aarch64-linux"
+        ];
+        imports = filter (hasSuffix ".ulu.nix") (listFilesRecursive ./nix);
+      }
+    );
+}

nix/devshell.ulu.nix

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+{ ... }:
+{
+  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"
+        '';
+      };
+    };
+}

nix/formatter.ulu.nix

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+{ ... }:
+{
+  perSystem =
+    { pkgs, ... }:
+    {
+      formatter = pkgs.writeShellApplication {
+        name = "nixfmt-tree";
+        runtimeInputs = [
+          pkgs.nixfmt
+          pkgs.fd
+          pkgs.findutils
+        ];
+        text = ''
+          if [ "$#" -eq 0 ]; then
+            set -- .
+          fi
+          fd --type f --extension nix --hidden --exclude .jj . "$@" \
+            | xargs -r nixfmt
+        '';
+      };
+    };
+}

nix/package.ulu.nix

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+{ ... }:
+{
+  perSystem =
+    { pkgs, lib, ... }:
+    {
+      packages.default = pkgs.rustPlatform.buildRustPackage {
+        pname = "valw";
+        version = "0.1.0";
+
+        src = lib.cleanSourceWith {
+          src = ../.;
+          filter =
+            path: _type:
+            !(builtins.elem (baseNameOf path) [
+              ".direnv"
+              ".jj"
+              "result"
+              "target"
+            ]);
+        };
+
+        cargoLock.lockFile = ../Cargo.lock;
+
+        nativeBuildInputs = [ pkgs.pkg-config ];
+        buildInputs = [
+          pkgs.libxkbcommon
+          pkgs.wayland
+        ];
+
+        # Tests run through `nix flake check` instead.
+        doCheck = false;
+
+        meta = {
+          description = "macOS-style screenshots for niri";
+          mainProgram = "valw";
+          platforms = lib.platforms.linux;
+        };
+      };
+    };
+}

src/main.rs

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+// Modules land before the CLI uses them; Task 11 removes this.
+#![allow(dead_code)]
+
+use clap::Parser;
+
+#[derive(Parser)]
+#[command(version, about = "macOS-style screenshots for niri")]
+struct Cli {}
+
+fn main() {
+    Cli::parse();
+}