feat(mental-health): measure screener change against the meaningful-change threshold

The assessments list now carries, per PHQ-9, GAD-7 and WHO-5, the latest
total against the previous and the first administration, classified
against the change studies treat as meaningful: 5 points on the PHQ-9,
4 on the GAD-7, 10 on the WHO-5 percentage scale. A smaller change reads
as within the threshold rather than better or worse. The instrument
detail states each change and whether it reaches that size, in neutral
words. The read takes totals and dates only, never an answer.

src/__tests__/analysis-depth-no-ai-guard.test.ts

@@ -30,6 +30,7 @@ const ANALYSIS_MODULES = [
   "lib/insights/discovery-matrix.ts",
   "lib/insights/derived/changepoint.ts",
   "lib/labs/lab-trend.ts",
+  "lib/mental-health/assessment-trend.ts",
 ] as const;
 
 /** Reachable under `lib/ai/` without being a model: the at-rest cipher. */