Derive: recalculate a night from where the last sync stopped, and survive an app restart

lib/app.dart

@@ -76,10 +76,16 @@ class _OpenStrapAppState extends State<OpenStrapApp> with WidgetsBindingObserver
         WidgetsBinding.instance.platformDispatcher.platformBrightness);
   }
 
+  @override
+  void didHaveMemoryPressure() {
+    context.read<AppState>().handleMemoryPressure();
+  }
+
   @override
   void didChangeAppLifecycleState(AppLifecycleState state) {
     final app = context.read<AppState>();
     if (state == AppLifecycleState.resumed) {
+      app.resumeDerivationRetention();
       // The user may have flipped our notification switch either way in OS
       // Settings while we were backgrounded. Drop the cached authorization
       // decision so the next present/schedule re-reads reality — a denial used

lib/compute/cardio_rows_cache.dart

@@ -0,0 +1,46 @@
+import 'dart:convert';
+import 'dart:typed_data';
+
+import 'package:openstrap_analytics/onehz.dart' as ana;
+
+/// Caller-owned bytes and provenance. Row objects exist only in the worker.
+class CardioRowsChunk {
+  CardioRowsChunk(this.bytes, this.loSec, this.hiSec, this.revisions,
+      this.generation, this.algorithm);
+  final Uint8List bytes;
+  final int loSec, hiSec;
+  final Map<String, (int, int, int)> revisions;
+  int generation, algorithm;
+
+  bool overlaps(int lo, int hi) => hiSec >= lo && loSec <= hi;
+
+  bool matches(Map<String, (int, int, int)> current, int gen, int algo) =>
+      generation == gen && algorithm == algo &&
+      current.entries.every((e) =>
+          !revisions.containsKey(e.key) || revisions[e.key] == e.value);
+}
+
+/// Worker only. Damage or an unknown codec version costs a cold staging.
+ana.CardioRowCache decodeCardioRows(List<Uint8List> chunks) {
+  final rows = ana.CardioRowCache();
+  for (final bytes in chunks) {
+    try {
+      rows.merge(ana.CardioRowCache.fromJson(
+          (jsonDecode(utf8.decode(bytes)) as Map).cast<String, dynamic>()));
+    } on FormatException {
+      // Refuse the whole chunk.
+    } on TypeError {
+      // A structurally invalid JSON value is also a miss.
+    }
+  }
+  return rows;
+}
+
+/// Keep at most one search window plus slack and 8,000 row variants. A limit
+/// only evicts reusable work; analytics always recomputes a missing row.
+Uint8List encodeCardioRows(ana.CardioRowCache rows, int lo, int hi) {
+  var kept = 0;
+  rows.retainWhere((k) =>
+      k.startSec >= lo && k.startSec <= hi && kept++ < 8000);
+  return Uint8List.fromList(utf8.encode(jsonEncode(rows.toJson())));
+}