RetrieX Generic Parameter Role Guard Fix

This patch keeps product-selection grounding generic while fixing parameter/accessory side effects.

Core idea:

Role guard stays generic: main-device requests must not be answered with accessories, and explicit accessory requests must not be turned into main-device requests.
Measurement evidence is context-sensitive: a parameter mention counts only when it is tied to measurement purpose such as measurement, measuring range, measurement parameter, determination, analysis, indicator/reagent for that parameter, sensor, or electrode.
Operating conditions, sample/environment limits, reagent-solution pH values, and pH values at a temperature are not sufficient evidence by themselves.

This patch does not touch retrieval, scoring, SSE, job lifecycle, or LLM timeout logic.