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

internal/examples/effectspike/main.go

@@ -0,0 +1,52 @@
+// Spike to validate that Datastar's data-effect re-evaluates when signals are
+// updated via PatchSignals from the server, and that Via's hex signal IDs work
+// in $signalID expression syntax.
+package main
+
+import (
+	"fmt"
+	"math/rand"
+	"time"
+
+	"github.com/ryanhamamura/via"
+	"github.com/ryanhamamura/via/h"
+)
+
+func main() {
+	v := via.New()
+	v.Config(via.Options{
+		DocumentTitle: "data-effect Spike",
+		ServerAddress: ":7332",
+		DevMode:       true,
+	})
+
+	v.Page("/", func(c *via.Context) {
+		x := c.Signal(0)
+		y := c.Signal(0)
+
+		c.OnInterval(time.Second, func() {
+			x.SetValue(rand.Intn(500))
+			y.SetValue(rand.Intn(500))
+			c.SyncSignals()
+		})
+
+		c.View(func() h.H {
+			return h.Div(
+				h.Attr("style", "padding:1rem;font-family:sans-serif"),
+				h.H1(h.Text("data-effect Spike")),
+				h.P(h.Text("x: "), x.Text(), h.Text(" y: "), y.Text()),
+				h.Div(
+					h.ID("box"),
+					h.Attr("style", "width:20px;height:20px;background:red;position:absolute"),
+					h.DataEffect(fmt.Sprintf(
+						"document.getElementById('box').style.left=$%s+'px';"+
+							"document.getElementById('box').style.top=$%s+'px'",
+						x.ID(), y.ID(),
+					)),
+				),
+			)
+		})
+	})
+
+	v.Start()
+}