P2: ANN/non-ANN vector performance — tests & benchmarks

[ZanzyTHEbar]

Context: Validate ANN vector_top_k vs exact ORDER BY distance path across correctness, pagination/ordering, and performance (micro-bench guidance).

Acceptance criteria:

• Deterministic corpus for vector similarity tests.
• Tests cover vector_top_k path and ORDER BY fallback, including pagination and ordering.
• Micro-bench or guidance notes documented (optional).