Update Measurement

Partial update. Geometry triggers server-side recompute of computed_value; extension_values are validated against the condition's attached extension at PATCH time.

extra="forbid" so attempts to PATCH condition_id, page_id, or computed_value get a clean 422 instead of being silently dropped.

Every measurement PATCH returns this: one item for an ordinary measurement, N in member id order for a link group (RUD-263 spec §7.1).

One stable envelope rather than two shapes — pre-launch, the clean break beats a compat shim that every future caller would have to remember.