Fix stale spec version in CONTRIBUTING.md (v0.4 -> v0.6)

Co-Authored-By: Claude Fable 5 <[email protected]>

CONTRIBUTING.md

@@ -51,7 +51,7 @@ reachable from most contributor sandboxes either.
 
 ## The spec is canonical
 
-`docs/spec.md` (currently v0.4) is the source of truth for the protocol
+`docs/spec.md` (currently v0.6) is the source of truth for the protocol
 itself — wire formats, KDF parameters, capsule structure, derivation paths,
 threat model. Code comments reference it by section (`§11.2`, `§17.4`, …).