* fix: guard against top_k=0 reaching the vector store
Turbopuffer rejects top_k=0 with a 400 ('top_k must be between 1 and
10000'). The fix returns [] for a non-positive top_k, before the embedding call), and
floor the semantic budget at 1 so an explicitly requested search isn't
silently allocated zero.
* fix: comments
148f646796