V3-11: offline tile pre-download along a route corridor

Three pure modules: tile_math.dart (tilesForBounds/tilesAlongRoute, hard cap enforced by throwing), tile_cache.dart (FileTileCache with LRU eviction before any write that would exceed the cap), tile_downloader.dart (sequential, rate-limited, cooperatively cancellable, one bad tile doesn't abort the rest). CachedTileProvider wires the cache into flutter_map via a custom ImageProvider and now backs RideMap and RoutePlannerScreen's tile layers, so ordinary viewing write-throughs into the same capped cache.

RoutePlannerScreen gained a route-corridor download action (no rectangle-selection UI -- the ticket names the corridor as strictly better and V3-07 already exists to hang it off of), with a count/size confirmation before any request and a cancellable progress dialog. Fixed a real bug before shipping: a StatefulBuilder-based progress dialog would have started a new overlapping download subscription on every single progress tick; moved to a dedicated StatefulWidget that subscribes once in initState.

Marked partially done: aeroplane-mode verification on a real device is the ticket's own acceptance criterion and needs a phone this environment doesn't have.

lib/src/ui/components/ride_map.dart

@@ -44,12 +44,18 @@ class RideMap extends StatefulWidget {
     required this.segments,
     this.height = 320,
     this.follow = false,
+    this.tileProvider,
   });
 
   final List<TrackPoint> points;
   final List<Segment> segments;
   final double height;
 
+  /// V3-11: when supplied, tiles are read from (and written through to) the offline
+  /// tile cache instead of flutter_map's own uncapped default. Null falls back to
+  /// ordinary networked tiles -- used whenever the cache isn't ready yet, or in tests.
+  final TileProvider? tileProvider;
+
   /// V3-04: keep the latest point centred while recording. A manual pan/pinch turns
   /// this off until the widget is rebuilt fresh (e.g. a new ride) — chasing the rider
   /// back to centre after they deliberately looked elsewhere would be worse than not
@@ -184,6 +190,7 @@ class _RideMapState extends State<RideMap> with WidgetsBindingObserver {
                 maxNativeZoom: maxTileZoom.toInt(),
                 // Respect OSM's usage policy: render what is looked at, never bulk prefetch.
                 panBuffer: 0,
+                tileProvider: widget.tileProvider,
               ),
             PolylineLayer(polylines: polylines),
           ],