feat(insights): show the metric explainer inline under each category heading

The per-category explainer text only lived behind the "?" popover next to
the heading. Render the same localised definition inline as a muted caption
directly beneath the title, where the short scaffold line already sits, so
the one- to two-sentence definition is discoverable without a tap.

Both surfaces read the identical insights.subPage.explainer.<metric>Body
key, so there is no duplicated copy and all six locales stay covered. The
"?" glyph stays in place as the always-available pointer to the same text.
HealthkitMetricPage already forwards explainerMetric to the shell, so every
routed metric sub-page picks up the inline caption from this one change.

src/components/insights/__tests__/sub-page-shell.test.tsx

@@ -47,7 +47,30 @@ describe("<SubPageShell>", () => {
       </SubPageShell>,
     );
     // The shell uses a `<p>` for the description; no `<p>` appears when
-    // the description prop is absent.
+    // neither a description nor an explainer metric is supplied.
     expect(html).not.toMatch(/<p[^>]*class="text-muted-foreground/);
   });
+
+  it("renders the explainer body inline beneath the heading", () => {
+    const html = render(
+      <SubPageShell title="Blood pressure" explainerMetric="bloodPressure">
+        <span />
+      </SubPageShell>,
+    );
+    // The inline caption reuses the same body string the `?` popover reads.
+    expect(html).toMatch(/data-slot="metric-explainer-inline"/);
+    expect(html).toContain(
+      "Blood pressure is the force your blood exerts on the artery walls",
+    );
+  });
+
+  it("keeps the explainer trigger glyph when explainerMetric is set", () => {
+    const html = render(
+      <SubPageShell title="Blood pressure" explainerMetric="bloodPressure">
+        <span />
+      </SubPageShell>,
+    );
+    // The `?` glyph stays available alongside the inline caption.
+    expect(html).toMatch(/data-slot="metric-explainer-trigger"/);
+  });
 });

src/components/insights/sub-page-shell.tsx

@@ -5,6 +5,7 @@ import { useEffect, useRef, type ReactNode } from "react";
 import { Badge } from "@/components/ui/badge";
 import { MetricExplainer } from "@/components/insights/metric-explainer";
 import { useScrollResetOnRoute } from "@/hooks/use-scroll-reset-on-route";
+import { useTranslations } from "@/lib/i18n/context";
 import { cn } from "@/lib/utils";
 
 /**
@@ -41,6 +42,13 @@ export interface SubPageShellProps {
    * The value is the metric key feeding
    * `insights.subPage.explainer.<metric>{Title,Body}`. Omitted on the
    * mother page; every routed metric sub-page passes its key.
+   *
+   * v1.8.4 — the same explainer body also renders inline as a muted
+   * caption directly beneath the heading, so the one- to two-sentence
+   * definition is visible without opening the popover. The `?` glyph
+   * stays as the always-available pointer for the same copy. Both the
+   * inline caption and the popover read the identical
+   * `insights.subPage.explainer.<metric>Body` string — no duplicated copy.
    */
   explainerMetric?: string;
   /**
@@ -69,6 +77,7 @@ export function SubPageShell({
   focusOnMount = false,
   children,
 }: SubPageShellProps) {
+  const { t } = useTranslations();
   // a11y: focus the heading on mount so a tab-strip navigation actually
   // moves screen-reader focus into the sub-page body. v1.4.27 MB7 /
   // CF-35 made the focus call opt-in via `focusOnMount` — the
@@ -118,6 +127,18 @@ export function SubPageShell({
             </Badge>
           ) : null}
         </div>
+        {explainerMetric ? (
+          // v1.8.4 — surface the explainer definition inline, reusing the
+          // exact body string the `?` popover reads. Muted caption styling
+          // matches the description paragraph below so the under-heading
+          // band reads as one calm block.
+          <p
+            data-slot="metric-explainer-inline"
+            className="text-muted-foreground text-sm leading-relaxed"
+          >
+            {t(`insights.subPage.explainer.${explainerMetric}Body`)}
+          </p>
+        ) : null}
         {description ? (
           <p className="text-muted-foreground text-sm">{description}</p>
         ) : null}