Compute a single festival in a given year for a given city. Useful for 'When is Diwali 2026 in Sydney?' style questions.

The 200 occurrence payload is read-only and deterministic and is fetched by answer-engine/SEO crawlers for 'when is <festival> <year>' queries, so it is gzip-negotiated via the audited _json_response_maybe_gzip helper: the DECOMPRESSED body is byte-identical to the old return _with_meta({...}) path (no contract/field/value change), and both 404 paths (unknown rule key, un-resolvable occurrence) are unchanged. No PII; no reflected request input.