Add RotationSignal field to Marker so rotation updates reactively via
signals, matching LngSignal/LatSignal. The markerEffectExpr now calls
setRotation() when RotationSignal is present.
Fix ship orientation in the example by subtracting 90 degrees from the
north-based heading to account for the east-facing SVG bow.
Provides a Go API for interactive maps within Via applications:
- Plugin serves vendored MapLibre GL JS v4.7.1 assets
- Map struct with pre/post-render source, layer, marker, popup management
- Viewport signal sync (center, zoom, bearing, pitch) via hidden inputs
- FlyTo, SetCenter, SetZoom and other viewport setters via ExecScript
- Idempotent init script with SPA cleanup via MutationObserver
- Example app demonstrating markers, GeoJSON layers, and FlyTo actions
