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.

Cargo.toml

@@ -15,9 +15,11 @@ path = "src/main.rs"
 name = "nordify"
 
 [dependencies]
+argh = "0.1.19"
 image = "0.25.10"
 ndarray = { version = "0.17.2", features = ["rayon"] }
 rayon = "1.12.0"
+thiserror = "2.0.20"
 wide = "1.6.1"
 
 [profile.release]