feat: refactor maplibre for reactive signals and Via-native API

- Export Signal type (signal → Signal) so subpackages can reference it
- Expose viewport signals as public fields (CenterLng, CenterLat, Zoom,
  Bearing, Pitch) for .Text() display and .Bind() usage
- Add signal-backed marker positions (LngSignal/LatSignal) with
  data-effect reactivity for server push and dragend writeback
- Add event system (MapEvent, OnClick, OnLayerClick, OnMouseMove,
  OnContextMenu) using hidden inputs + action triggers
- Add Source interface replacing type-switch, with RawSource escape hatch
- Add CameraOptions for FlyTo/EaseTo/JumpTo/FitBounds/Stop
- Add Control interface with NavigationControl, ScaleControl,
  GeolocateControl, FullscreenControl
- Expand Options with interaction toggles, MaxBounds, and Extra map
- Add effectspike example to validate data-effect with server-pushed signals
- Update maplibre example to showcase all new features

field.go

@@ -1,10 +1,10 @@
 package via
 
 // Field is a signal with built-in validation rules and error state.
-// It embeds *signal, so all signal methods (Bind, String, Int, Bool, SetValue, Text, ID)
+// It embeds *Signal, so all signal methods (Bind, String, Int, Bool, SetValue, Text, ID)
 // work transparently.
 type Field struct {
-	*signal
+	*Signal
 	rules      []Rule
 	errors     []string
 	initialVal any