fix: prevent RangeError in Proj4Crs.zoom for out-of-range scales

lib/src/geo/crs.dart

@@ -351,6 +351,14 @@ class Proj4Crs extends Crs {
     if (scale == downScale) {
       return downZoom.toDouble();
     }
+    // Scale is larger than the largest defined level: the requested zoom is
+    // beyond the deepest level this CRS knows about. Mirror the
+    // [double.negativeInfinity] returned above so callers (e.g.
+    // [FitBounds._getBoundsZoom]) can clamp to [maxZoom] rather than crashing
+    // on an out-of-range index access.
+    if (downZoom == _scales.length - 1) {
+      return double.infinity;
+    }
     // Interpolate
     final nextZoom = downZoom + 1;
     final nextScale = _scales[nextZoom];

test/geo/crs_test.dart

@@ -0,0 +1,40 @@
+import 'package:flutter_map/flutter_map.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:proj4dart/proj4dart.dart' as proj4;
+
+void main() {
+  group('Proj4Crs.zoom', () {
+    final crs = Proj4Crs.fromFactory(
+      code: 'EPSG:3978',
+      proj4Projection: proj4.Projection.add(
+        'EPSG:3978',
+        '+proj=lcc +lat_0=49 +lon_0=-95 +lat_1=49 +lat_2=77 +x_0=0 +y_0=0 '
+            '+ellps=GRS80 +units=m +no_defs',
+      ),
+      resolutions: const [4096.0, 2048.0, 1024.0, 512.0, 256.0],
+    );
+
+    test('returns the array index when the scale matches a level exactly', () {
+      // Scale at zoom n equals 1 / resolutions[n].
+      expect(crs.zoom(1 / 4096.0), 0);
+      expect(crs.zoom(1 / 256.0), 4);
+    });
+
+    test('interpolates between adjacent levels', () {
+      final lo = 1 / 2048.0;
+      final hi = 1 / 1024.0;
+      expect(crs.zoom((lo + hi) / 2), closeTo(1.5, 1e-9));
+    });
+
+    test('returns -infinity when scale is below the coarsest level', () {
+      expect(crs.zoom(1 / 8192.0), double.negativeInfinity);
+    });
+
+    test('returns +infinity when scale exceeds the finest level', () {
+      // Regression test for fleaflet/flutter_map#1223: before the fix this
+      // threw RangeError when indexing _scales[length] during interpolation
+      // past the last level.
+      expect(crs.zoom(1 / 128.0), double.infinity);
+    });
+  });
+}