Function by_clause_is_bearer 

pub fn by_clause_is_bearer(
    by: &ByClause,
    actors: &HashMap<String, ActorDecl>,
) -> bool

Resolve a handler’s Bearer seam, if its by clause names a local Bearer actor — or a refinement of one (v0.53), following the refinement to its base for the scheme/secret/identity and carrying the authorisation predicate. Returns None for non-Bearer handlers (prelude actors are never Bearer) — those emit unchanged. #706: whether a by clause names a Bearer-secured actor (following a refinement to its base). The routes for which by Nobody can drive the auth seam to a 401 — an unsecured (Visitor/None) or Signature/Oidc route has no Bearer seam the test driver knows how to leave unauthenticated, so by Nobody there is rejected (bynk.test.nobody_needs_secured_route). The scheme check mirrors bearer_seam_for.