Misc fixes

[davxy]

• Add ContextVec<T> (inline-3, spills to heap, capped at MAX_CONTEXTS on decode) with SCALE codec impls. AliasVec = ContextVec<Alias>. validate_multi_context / create_multi_context return it - the
  default single-context wrappers no longer heap-allocate.
• Unify on a single MAX_CONTEXTS = 16. Drop RingSuiteExt::MAX_VRF_CONTEXTS and the mock's local bound.
• Collapse RingVrfOutputs Single/Multi into a newtype around ContextVec; the inline/heap split is now ContextVec's job.
• MockProof.aliases uses ContextVec<Alias> instead of BoundedVec. Wire format unchanged.
• Guard create_multi_context against contexts.len() > MAX_CONTEXTS.