Return the engine's machine-readable VOCABULARY: every indicator type (with its multi-output selectors + primary params/defaults), the built-in feature ids, the benchmark-return operands, and the minute-only features.

UNAUTHENTICATED by design — this is public, static engine metadata (the same vocabulary llms.txt documents), not user data. The no-code builder fetches it once so its operand/indicator pickers speak the CURRENT engine spec instead of a hand-curated list that silently drifts. Served from a dict frozen at import (build_registry) — no per-request work, cacheable.