docs(coach): say what the labs block reads since it covers the whole record

The labs block reads the whole record, or up to the lookback limit, not just
recent readings, and a named read also comes back empty when no marker
matches. The header of the labs snapshot and the note at its call site
still described the earlier twelve-month window.

src/lib/ai/coach/labs-snapshot.ts

@@ -48,8 +48,9 @@
  * `userId` is narrowed from the authenticated session by the caller and feeds
  * the Prisma `where` field-by-field; it is never an input. Unlike the illness
  * block, Labs is intentionally NOT module-gated (mirrors `/api/labs` — the data
- * is always owner-scoped and safe to read), so the block is `null` only when the
- * account has no recent readings.
+ * is always owner-scoped and safe to read). The block is `null` only when no
+ * reading falls inside the lookback limit (the whole record when there is
+ * none), or, for a named read, when no marker matches the analyte.
  */
 import { sameLabUnit } from "@/lib/labs/unit-normalise";
 import {

src/lib/ai/coach/snapshot.ts

@@ -668,8 +668,8 @@ async function buildCoachSnapshotImpl(
   // scope-gated metric: attached without a `registerBlock` so the budget
   // degrader never sheds it, and Labs is intentionally not module-gated (the
   // helper reads owner-scoped rows directly, mirroring `/api/labs`). The block
-  // carries the most-recent resolved reading per biomarker (last 12 months,
-  // capped) so the Coach can answer "what was my LDL" without re-deriving.
+  // carries the latest resolved reading per biomarker (whole record or the
+  // lookback limit, capped) so the Coach can answer "what was my LDL".
   const labsBlockPromise = buildLabsSnapshotBlock(userId, now, reach);
 
   // v1.38 — doctor-visit context. Like illness/labs it is attempted always (a