Propagate @Nullable across override relationships

Propagates existing JSpecify @Nullable annotations across @Override relationships so overrides honor their supertype's nullability contract. A @Nullable return propagates up to the overridden supertype method (returns are covariant); a @Nullable parameter propagates down to every overriding method (parameters are contravariant). Methods are matched across files by erased signature name(paramTypes) plus a declaring-type subtype relationship. Conservative by design: it never widens a legal covariant return narrowing, skips primitive/void returns and already-annotated positions, and does nothing when a participating type cannot be resolved. Only Java sources are modified.