add TFL bus stop caching and fetching functionality, enhance map rendering with bus stop markers

lib/pages/map/vector/mvt_parser.dart

@@ -1,3 +1,4 @@
+import 'dart:convert';
 import 'dart:typed_data';
 import 'dart:ui';
 
@@ -316,7 +317,7 @@ class _PbfReader {
 
   String readString() {
     final bytes = readBytes();
-    return String.fromCharCodes(bytes);
+    return utf8.decode(bytes, allowMalformed: true);
   }
 
   double readFloat32() {

lib/pages/map/vector/tile_pyramid_cache.dart

@@ -28,7 +28,7 @@ class TilePyramidCache {
   TilePyramidCache._();
 
   static const int _maxEntries = 384;
-  static const int _maxApproxBytes = 256 * 1024 * 1024;
+  static const int _maxApproxBytes = 768 * 1024 * 1024;
 
   // Cap concurrent renders to avoid saturating the main isolate with
   // synchronous canvas recording. I/O (network/Hive) is still concurrent