refactor(healthkit): fold the per-type arrival tally into the ledger module

The batch route counted arrivals inline and grew past its size baseline
with the new-sample rule. tallyHealthKitArrivals now owns the fold, the
route hands it each landed entry and hour with whether it brought a new
value, and the size baselines move down to what the route is now.

Refs #1259

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": 410,
-  "total": 148056,
+  "total": 148049,
   "entries": {
     "app/achievements/page.tsx :: AchievementsPage": 206,
     "app/api/admin/backups/upload/route.ts :: apiHandler(arg)": 314,
@@ -38,7 +38,7 @@
     "app/api/mcp/oauth/token/route.ts :: POST > withBackgroundEvent(arg)": 248,
     "app/api/measurement-reminders/[id]/route.ts :: apiHandler(arg) #2": 240,
     "app/api/measurements/[id]/route.ts :: apiHandler(arg) #2": 203,
-    "app/api/measurements/batch/route.ts :: postBatch": 1262,
+    "app/api/measurements/batch/route.ts :: postBatch": 1255,
     "app/api/measurements/route.ts :: apiHandler(arg)": 286,
     "app/api/measurements/route.ts :: postMeasurement": 377,
     "app/api/measurements/series/route.ts :: apiHandler(arg)": 549,