feat: wire up image conversion pipeline and CLI

Add `convert.rs`: a SIMD nearest-color search and a rayon-parallel
`nordify_pixels` function. Add `error.rs` with the `NordifyError` type.

Rewrite `lib.rs` to expose `NordifyOptions`, `nordify_image`, and
`nordify_file` as the public API. Rewrite `main.rs` to parse CLI
arguments with `argh` and call into `lib.rs`.

Add `argh` and `thiserror` as dependencies.

src/main.rs

@@ -1,3 +1,45 @@
-fn main() {
-    println!("Hello, world!");
+use std::path::PathBuf;
+
+/// Convert an image to the Nord color palette without dithering artifacts.
+#[derive(argh::FromArgs)]
+struct Args {
+    /// input image path
+    #[argh(positional)]
+    input: PathBuf,
+
+    /// output image path
+    #[argh(option, short = 'o')]
+    output: PathBuf,
+
+    /// use only the original 16 Nord colors (disable palette expansion)
+    #[argh(switch)]
+    no_expand: bool,
+
+    /// number of interpolated colors between close palette entries (default: 3)
+    #[argh(option, default = "3")]
+    expansion_factor: usize,
+
+    /// use plain RGB distance instead of perceptual LAB distance
+    #[argh(switch)]
+    rgb_distance: bool,
+}
+
+fn main() -> std::process::ExitCode {
+    let args: Args = argh::from_env();
+    let options = nordify_rs::NordifyOptions {
+        expand: !args.no_expand,
+        expansion_factor: args.expansion_factor,
+        use_lab: !args.rgb_distance,
+    };
+
+    match nordify_rs::nordify_file(&args.input, &args.output, &options) {
+        Ok(()) => {
+            println!("Saved to {}", args.output.display());
+            std::process::ExitCode::SUCCESS
+        }
+        Err(err) => {
+            eprintln!("Error: {err}");
+            std::process::ExitCode::FAILURE
+        }
+    }
 }