feat: add color module with sRGB to Lab conversion (scalar + SIMD)

- src/color.rs: Rgb/Lab types, planar SoA storage, wide-based SIMD
  RGB to Lab conversion with scalar remainder tail
- src/palette.rs: NORD_PALETTE constant
- Cargo.toml: add image/ndarray/rayon/wide deps, tuned release profile

Cargo.toml

@@ -15,3 +15,11 @@ path = "src/main.rs"
 name = "nordify"
 
 [dependencies]
+image = "0.25.10"
+ndarray = { version = "0.17.2", features = ["rayon"] }
+rayon = "1.12.0"
+wide = "1.6.1"
+
+[profile.release]
+lto = true
+codegen-units = 1