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/detail/trip_detail_screen.dart

@@ -326,7 +326,13 @@ class _Body extends StatelessWidget {
         ),
         const SizedBox(height: 24),
         if (detail.mapEnabled)
-          RideMap(points: detail.points, segments: detail.segments),
+          Consumer(
+            builder: (context, ref, _) => RideMap(
+              points: detail.points,
+              segments: detail.segments,
+              tileProvider: ref.watch(cachedTileProviderProvider),
+            ),
+          ),
         if (detail.mapEnabled) const SizedBox(height: 24),
         Card(
           child: Padding(