refactor(charts): split HealthChart into its read, view, layers and header

HealthChart was a 1,929-line component. Its parts move to
components/charts/health-chart/:

- series-math: series names, day keys, moving average, personal
  baseline, annotation and marker placement (pure)
- use-health-chart-series: the fetch window, the batched-slice decision,
  the daily read and the data-ready latch
- use-health-chart-view: the visible slice, bucketing, the average and
  trend lines, the prior period, the visible-range stats and the y domain
- use-health-chart-overlays: value bands, target zones, baselines, the
  trend read-out, the spoken summary and the table columns
- chart-header, band-overlay, chart-tooltip-content
- chart-layers: the reference layers and series as plain functions that
  return the Recharts elements, so the plot's child list is unchanged

health-chart.tsx keeps the component and its path (chart-runtime still
imports it from there) and shrinks to 488 lines. Output is unchanged.
The guards pinned to the file follow the code into the folder, and the
baseline-gating check now calls the layer instead of matching source.

src/__tests__/baselines/source-function-lines.json

@@ -1,7 +1,7 @@
 {
   "$comment": "Production functions over 200 lines, keyed \"file :: name\", and their length. Maintained by src/__tests__/source-size-ratchet.test.ts.",
   "count": 408,
-  "total": 144450,
+  "total": 143009,
   "entries": {
     "app/achievements/page.tsx :: AchievementsPage": 206,
     "app/api/admin/backups/upload/route.ts :: apiHandler(arg)": 314,
@@ -97,7 +97,7 @@
     "components/auth/claim-profile.tsx :: ClaimProfile": 429,
     "components/auth/mfa-login-step.tsx :: MfaLoginStep": 217,
     "components/charts/compliance-heatmap.tsx :: ComplianceHeatmap": 325,
-    "components/charts/health-chart.tsx :: HealthChart": 1929,
+    "components/charts/health-chart.tsx :: HealthChart": 488,
     "components/charts/medication-compliance-chart.tsx :: MedicationComplianceChart": 543,
     "components/charts/mood-chart.tsx :: MoodChart": 896,
     "components/charts/mood-heatmap.tsx :: MoodHeatmap": 296,