queryVector(): setRadius() semantics for METRIC_IP are non-intuitive (score vs internal distance)

[s2x]

Issue (found during #197 fix, verified by subagent review)

Radius is compared against the internal distance representation (score <= threshold), so for inner-product metric (METRIC_IP) probes radius values 0.4–1.0 never filter anything. Users setting setRadius() on IP collections may silently get unfiltered results.

Evidence

• zvec/src/include/zvec/core/framework/index_document.h:276 — score <= threshold_
• Radius semantics for IP metric differ from cosine/L2 where a distance threshold makes intuitive sense.

Suggested action

• Likely upstream documentation/design concern (alibaba/zvec).
• In this repo: document the radius semantics per metric in the PHP docs / examples, and/or add a validation warning when setRadius() is used with a METRIC_IP collection.