Add plugins/maplibre, a MapLibre GL JS v5 plugin mirroring the echarts
design: the plugin injects the CDN JS + (required) CSS and a per-page map
registry, and a *Map drives the camera, markers, and GeoJSON data layers
from Go over SSE.

- Construction: center/zoom/pitch/bearing, zoom range, max bounds,
  navigation/scale/geolocate/fullscreen controls, GeoJSON sources + layers,
  and a WithMapOption escape hatch.
- Camera: FlyTo/EaseTo/JumpTo/SetCenter/SetZoom/SetPitch/SetBearing/FitBounds.
- Markers: keyed AddMarker/MoveMarker/RemoveMarker/ClearMarkers with
  Color/Draggable/Scale and XSS-safe PopupText (plus trusted-only PopupHTML).
- Data: GeoJSON + layer builders; SetGeoJSON/AddGeoJSONSource/AddLayer/
  SetPaintProperty/SetLayerVisibility/RemoveLayer(AndSource), guarded by a
  re-arming style-load wait.
- Lifecycle: SetStyle/Resize/Dispose and a Call escape hatch.

Add internal/examples/maps (server-driven world map) and document the
plugin in docs/plugins.md.

…coords

Build the plugin into a full interactive-map toolkit, all driven from Go.

Inbound events: OnClick, OnMoveEnd, OnMarkerClick, OnMarkerDragEnd,
OnFeatureClick, and a generic OnMapEvent escape hatch route browser gestures
to bound methods, read via Map.Event(ctx). Handlers use the via.Action
func-union constraint (no public `any`). Every gesture carries the live camera
(zoom/bearing/pitch). WithFeatureHover highlights features client-side via
feature-state; clickable markers/features show a pointer cursor.

Styling: typed expression builders (Get, FeatureState, Zoom, Boolean, Case,
Interpolate, Step) plus WhenHovered/WhenState sugar replace hand-written
[]any expression trees.

Coordinates: LngLat{Lng,Lat} + At() and Bounds{West,South,East,North} replace
bare float pairs to defuse the lng/lat and bbox swap foot-guns; MapEvent.LngLat()
flows a gesture straight into the camera/marker APIs. WithMarker declares static
markers at construction (no OnConnect); GenerateFeatureIDs assigns feature ids
for feature-state.

Dialogs: ShowPopup / ShowPopupHTML / ClosePopup show keyed, server-driven
popups. The HTML variant and marker PopupHTML take an h.H, so popup bodies
compose with the same h.* builders (h.T escapes user data; only h.Raw is raw).

Inbound-event signals use a non-underscore namespace (datastar drops
_-prefixed local signals). Multi-map pages are isolated by per-map registry
slots. Documented in doc.go and demonstrated in internal/examples/maps.