Animate the radar timeline with a double-buffered overlay

Completes milestone 3: frames render over the map, play as a loop, and can be
scrubbed, with the legend drawn from the manifest rather than a constant that
could drift from what the worker actually rendered.

The overlay alternates two MapLibre image sources. Updating one source in place
flickers, because the layer briefly shows a half-written texture; adding every
frame as its own layer avoids that but pins them all in GPU memory, and twenty
512x512 RGBA frames is about 80 MB. Two buffers cost the same whether the
timeline holds six frames or sixty. Platform-channel work is serialised because
`show` is called faster than the round trip completes during playback, and
overlapping updates would swap visibility out of order and strobe.

Prefetching loads a window around the playhead, nearest first and forward
before backward, since playback moves forward and that frame is needed
soonest. `FrameCache` is byte-budgeted rather than entry-counted because frame
size tracks how much precipitation is on screen, and it evicts by distance from
the playhead: plain LRU would keep frames the prefetcher touched a moment ago
even after the playhead moved to the far end of the timeline.

Also adds DpcRadarSource, which reads published frames from our CDN and never
from the DPC API. Without it, the `dpc` adapter would have had to fall back to
mock, putting demo frames on screen under the label of live data — exactly the
confusion the adapter split exists to prevent. It now fails naming the missing
setting instead.

Running it on the emulator caught three things the tests had not:

- The notifier wrote to `state` from inside `build()`, which Riverpod rejects
  as an uninitialised provider. That broke startup, not just tests.
- Eight-month-old demo frames rendered as "Aggiornato 342535 minuti fa". The
  age formatter now steps up to hours and days.
- Demo mode sat permanently behind a stale-data warning and so never showed the
  working state it exists to demonstrate. MockRadarSource now shifts the
  bundled timestamps onto the present, leaving images, order and spacing
  untouched, so the timeline behaves exactly as it would on live data.

Corrects docs/stack-decisions.md, which described a disk cache that was not
built: mock frames already live in the asset bundle, so a disk layer belongs
with the network adapter where it would save a real request.

Verified: analyze clean, 129 tests passing, and on the emulator the loop
advances, wraps, and reports "Aggiornato ora".

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Nuvolari/app/lib/data/radar/mock_radar_source.dart

@@ -16,13 +16,19 @@ import 'radar_source.dart';
 /// carry the same manifest shape the Python worker publishes, so nothing here
 /// is a special case.
 class MockRadarSource implements RadarSource {
-  MockRadarSource({this.bundle, this.manifestAsset = defaultManifestAsset});
+  MockRadarSource({
+    this.bundle,
+    this.manifestAsset = defaultManifestAsset,
+    DateTime Function()? clock,
+  }) : _clock = clock ?? DateTime.now;
 
   static const String defaultManifestAsset = 'assets/mock/manifest.json';
 
   /// Overridden in tests; null means the real asset bundle.
   final AssetBundle? bundle;
 
+  final DateTime Function() _clock;
+
   /// Asset path of the manifest; frame paths are resolved relative to it.
   final String manifestAsset;
 
@@ -48,7 +54,7 @@ class MockRadarSource implements RadarSource {
     }
 
     try {
-      return _cached = RadarManifest.parse(source);
+      return _cached = _rebaseToNow(RadarManifest.parse(source));
     } on FormatException catch (error) {
       // A malformed bundled asset is a packaging bug, but it still reaches the
       // caller as unavailability so demo mode degrades like any other source.
@@ -77,6 +83,52 @@ class MockRadarSource implements RadarSource {
     }
   }
 
+  /// Shifts the bundled timestamps so the newest frame lands on the present.
+  ///
+  /// The generator writes a fixed epoch so regenerating the assets produces
+  /// identical bytes and the committed frames do not churn. Left alone, demo
+  /// mode would therefore always sit behind a "data is stale" warning and never
+  /// show what the app looks like when things are working — which is the one
+  /// thing demo mode exists to show.
+  ///
+  /// Only the timestamps move. The images, their order and their spacing are
+  /// untouched, so the timeline behaves exactly as it would on live data.
+  RadarManifest _rebaseToNow(RadarManifest manifest) {
+    if (manifest.frames.isEmpty) return manifest;
+
+    final interval = manifest.frames.length >= 2
+        ? manifest.frames.last.timestamp.difference(
+            manifest.frames[manifest.frames.length - 2].timestamp,
+          )
+        : const Duration(minutes: 5);
+
+    // Land on a whole multiple of the interval rather than an arbitrary
+    // millisecond, the way a real publication would.
+    final now = _clock().toUtc();
+    final step = interval.inMilliseconds;
+    final target = DateTime.fromMillisecondsSinceEpoch(
+      step <= 0
+          ? now.millisecondsSinceEpoch
+          : now.millisecondsSinceEpoch - now.millisecondsSinceEpoch % step,
+      isUtc: true,
+    );
+
+    final shift = target.difference(manifest.frames.last.timestamp);
+
+    return RadarManifest(
+      regionId: manifest.regionId,
+      product: manifest.product,
+      generatedAt: manifest.generatedAt.add(shift),
+      bounds: manifest.bounds,
+      frames: List<RadarFrame>.unmodifiable(<RadarFrame>[
+        for (final frame in manifest.frames)
+          RadarFrame(timestamp: frame.timestamp.add(shift), path: frame.path),
+      ]),
+      legend: manifest.legend,
+      attribution: manifest.attribution,
+    );
+  }
+
   /// Resolves a frame's manifest-relative path against [manifestAsset].
   String resolveAssetPath(RadarFrame frame) {
     final separator = manifestAsset.lastIndexOf('/');