Provide a streaming executor #47

The executor is pure and synchronous over whole in-memory documents.

Decision: decline. encode and decode take a parsed JSON value, so by the time a caller has one the whole document is already in memory; a streaming executor would need a streaming JSON tokenizer and an incremental writer inside a platform-neutral library. The rules themselves are not stream-safe in general: a when reads arbitrary paths of the governing document (including later fields and collections), fixed indices and one-feeder arrays need the array's length, and the gap and conflict checks run over the finished output. A sequence of records (NDJSON, one document per line) already streams by calling encode per record, which needs nothing from the library.

Revisit when a single document does not fit in memory, starting from a stream-safe subset (wildcard-aligned rules whose when reads only the current element) behind a separate entry point.

The README reason for this omission is updated in #56.