feat(cargo): add rayon as optional dependency

Opt-in feature flag — users who want parallel paths build with
--features rayon. Default build remains single-threaded.

Spec Section 6 calls for default-on; we defer that flip until the
feature is stable under field use.

Part of T3 of docs/superpowers/specs/2026-04-23-trueskill-engine-redesign-design.md.

Cargo.toml

@@ -16,8 +16,13 @@ harness = false
 
 [dependencies]
 approx = { version = "0.5.1", optional = true }
+rayon = { version = "1", optional = true }
 smallvec = "1"
 
+[features]
+approx = ["dep:approx"]
+rayon = ["dep:rayon"]
+
 [dev-dependencies]
 criterion = "0.5"
 plotters = { version = "0.3", default-features = false, features = ["svg_backend", "all_elements", "all_series"] }