perf(prefetch): server-prefetch above-the-fold data for /insights, /medications, /coach

Extend the dashboard's RSC server-prefetch pattern to three more route roots
so their above-the-fold content paints populated on first load instead of
flashing a shell that fills in on mount.

Each route gains a thin RSC wrapper that runs the same cached read its API
route uses, dehydrates the result under the exact client query key, and hands
it to the client leaf through a HydrationBoundary. Every wrapper fails soft to
today's client-fetch path on any error and honours the DASHBOARD_SSR_PREFETCH
kill-switch the e2e server sets.

- /insights: prefetch the dashboard snapshot — the same locale-keyed cell the
  dashboard warms, resolved once server-side so the key matches by
  construction. The AI briefing and the heavy analytics aggregations stay
  client-lazy (the briefing must never trigger generation; a cold analytics
  read would trade FCP for TTFB).
- /medications: prefetch the medications list through the shared
  caches.medications cell (extracted to src/lib/medications/list-read.ts). The
  list cell keeps refetchOnMount: "always" — a cross-device take/skip produces
  no client mutation event, so the mount refetch is a correctness gate; the
  prefetch removes the initial skeleton flash without dropping that guarantee.
- /coach: prefetch the nudge status (extracted to
  src/lib/ai/coach/nudge-status.ts) so the auto-open-most-recent decision is
  ready at hydrate, collapsing the one sequential waterfall. Only runs when the
  Coach surface is reachable; the streaming conversation is never warmed.

Add per-route tests pinning server-key === client-key, JSON wire-shape
round-trip, and the fail-soft / module-gate paths. Repoint the source-reading
insights, take-all-due, and Coach gate-cascade tests at the client leaves.

src/components/medications/__tests__/take-all-due.test.tsx

@@ -508,7 +508,7 @@ describe("<TakeAllDueDialog> — confirm dialog lists the due medications", () =
     "utf8",
   );
   const pageSrc = readFileSync(
-    resolve(__dirname, "../../../app/medications/page.tsx"),
+    resolve(__dirname, "../../../app/medications/page-client.tsx"),
     "utf8",
   );