Covers: imprint_current base fallback, unload idempotency, load miss →
ok=false, ok field presence, respond passthrough for base/empty/unknown
IDs, graceful fallback after unload, surface_knowledge and
surface_memory_read return-type guarantees, base-scoped knowledge
equality, no-annotation invariant for base, empty-ID load rejection, and
failed-load state immutability.
Syntax follows El constraints: no Bool annotations, no &&/||, no unary !.
