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

computed_test.go

@@ -26,7 +26,7 @@ func TestComputedBasic(t *testing.T) {
 func TestComputedReactivity(t *testing.T) {
 	v := New()
 	var cs *computedSignal
-	var sig1 *signal
+	var sig1 *Signal
 	v.Page("/", func(c *Context) {
 		sig1 = c.Signal("a")
 		sig2 := c.Signal("b")
@@ -83,7 +83,7 @@ func TestComputedText(t *testing.T) {
 func TestComputedChangeDetection(t *testing.T) {
 	v := New()
 	var ctx *Context
-	var sig *signal
+	var sig *Signal
 	v.Page("/", func(c *Context) {
 		ctx = c
 		sig = c.Signal("a")