feat: serpentine map layout function

lib/ui/widgets/map_layout.dart

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+import 'dart:math' as math;
+import 'dart:ui';
+
+/// Deterministic serpentine layout for the journey map. Stage 0 is at the
+/// bottom; the path snakes upward. Works for any stage count.
+class MapLayout {
+  const MapLayout({
+    required this.width,
+    this.nodeSpacing = 108,
+    this.topPadding = 140,
+    this.bottomPadding = 150,
+  });
+
+  final double width;
+  final double nodeSpacing;
+  final double topPadding;
+  final double bottomPadding;
+
+  double get amplitude => width * 0.26;
+
+  double heightFor(int count) =>
+      topPadding + bottomPadding + (count - 1) * nodeSpacing;
+
+  Offset nodeCenter(int index, int count) {
+    final y = heightFor(count) - bottomPadding - index * nodeSpacing;
+    final x = width / 2 + amplitude * math.sin(index * 1.05);
+    return Offset(x, y);
+  }
+}

test/ui/map_layout_test.dart

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+import 'package:block_seasons/ui/widgets/map_layout.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  const layout = MapLayout(width: 400);
+
+  test('node 0 sits near the bottom, later nodes climb', () {
+    final h = layout.heightFor(60);
+    final first = layout.nodeCenter(0, 60);
+    final last = layout.nodeCenter(59, 60);
+    expect(first.dy, greaterThan(h - 200));
+    expect(last.dy, lessThan(200));
+    for (var i = 1; i < 60; i++) {
+      expect(layout.nodeCenter(i, 60).dy,
+          lessThan(layout.nodeCenter(i - 1, 60).dy));
+    }
+  });
+
+  test('x stays within horizontal margins', () {
+    for (var i = 0; i < 60; i++) {
+      final x = layout.nodeCenter(i, 60).dx;
+      expect(x, greaterThanOrEqualTo(400 * 0.12));
+      expect(x, lessThanOrEqualTo(400 * 0.88));
+    }
+  });
+
+  test('vertical spacing is uniform', () {
+    final a = layout.nodeCenter(3, 60).dy - layout.nodeCenter(4, 60).dy;
+    final b = layout.nodeCenter(40, 60).dy - layout.nodeCenter(41, 60).dy;
+    expect(a, closeTo(b, 0.001));
+  });
+}