fix(mood): say which age governs a claim about a dimension

The snapshot stamps every metric block with an asOf built from the
newest reading in it, which for mood is the newest ENTRY on the 1-5
axis. Somebody who taps a face each morning and opens the sliders once
a week therefore carries a block stamped as current and a stress
reading six days old, and a reader taking the block stamp as covering
the dimensions states last week's answer as today's.

The block now names the precedence in one line beside the dimensions,
and each dimension keeps its own age. Present only when there are
dimensions to explain.

src/lib/insights/features.ts

@@ -187,6 +187,27 @@ async function buildMoodDimensionFeatures(
   return blocks.length > 0 ? blocks : undefined;
 }
 
+/**
+ * The dimensions block and the note that says how to read it beside the
+ * block-level `asOf`. Absent entirely when nobody has answered a dimension,
+ * so an account that only ever taps a face carries neither key.
+ */
+async function buildMoodDimensionBlock(userId: string): Promise<{
+  dimensions?: MoodDimensionFeature[];
+  dimensionsAsOfNote?: string;
+}> {
+  const dimensions = await buildMoodDimensionFeatures(userId);
+  if (!dimensions) return {};
+  return {
+    dimensions,
+    dimensionsAsOfNote:
+      "The block-level asOf describes the newest mood ENTRY on the 1-5 axis. " +
+      "For the five dimensions below, each carries its own newestDaysAgo and " +
+      "current, and those are the ones that govern what may be said about " +
+      "them today; they are often older than the entry itself.",
+  };
+}
+
 export interface AggregatedFeatures {
   weight?: {
     latest: number;
@@ -295,6 +316,23 @@ export interface AggregatedFeatures {
      * given one age for the pair narrates the stale one as today's.
      */
     dimensions?: MoodDimensionFeature[];
+    /**
+     * Says which age governs which claim, because two of them ride in the
+     * same block and they disagree by design.
+     *
+     * `annotateSnapshotFreshness` stamps every metric block with an `asOf`
+     * built from `coverage.newestDaysAgo`, which for mood is the age of the
+     * newest ENTRY — the 1-5 axis. Somebody who taps a face each morning and
+     * opens the sliders once a week has `asOf.daysAgo` of 0 and a stress
+     * reading six days old, so a reader taking the block-level stamp as
+     * covering the dimensions would state last week's answer as today's.
+     * That is the exact sentence this milestone exists to prevent, and it is
+     * cheaper to name the precedence than to leave two true numbers looking
+     * like a contradiction.
+     *
+     * Present only alongside `dimensions`.
+     */
+    dimensionsAsOfNote?: string;
   };
   sleep?: {
     avg7: number | null;
@@ -1189,7 +1227,7 @@ export async function extractFeatures(
       latest: moodLatestScore,
       trend30,
       totalEntries: moodTotalEntries,
-      dimensions: await buildMoodDimensionFeatures(userId),
+      ...(await buildMoodDimensionBlock(userId)),
       coverage: {
         count: moodTotalEntries,
         spanDays,