[FEATURE] Add marker clustering for OpenStreetMap

[sfroemkenjw]

Resolves #169

Adds marker clustering for OpenStreetMap and makes the existing clustering setting provider-independent. This is an alternative to #377: instead of a separate OSM-only FlexForm setting, the existing site setting is reused, so clustering also works via TypoScript for maps rendered through the maps2 API without a FlexForm.

Changes

Settings

• Move site setting maps2.enableMarkerClusterer from site set Maps2 - Google Maps into Maps2 - Default Set. The key is unchanged, existing settings.yaml files keep working. It is now listed under category "Maps2 > Marker Clusterer".
• New FlexForm field settings.markerClusterer.override in tab "Map Options" for Google Maps and OpenStreetMap: use site setting / TypoScript (default, empty), enabled, disabled. An empty value never overrides TypoScript.
• New TypoScript setting plugin.tx_maps2.settings.markerClusterer.styleSheet (OSM only), default EXT:maps2/Resources/Public/Css/Leaflet/MarkerCluster.Default.css. Accepts EXT: and fileadmin/ paths and is included as-is via AssetCollector, no build step required. It is the OSM counterpart of markerClusterer.imagePath (GM only).

OpenStreetMap

• Clustering via the official Leaflet.markercluster ^1.5.3 (L.markerClusterGroup). Only point markers are clustered; areas, routes and radius stay on the map directly. No clustering in edit mode.
• showCoverageOnHover is disabled.
• Category switching works with the cluster group.
• Maps are initialized on DOMContentLoaded, so the cluster library is available even if only a later plugin on the page enables clustering.

Google Maps

• Bugfix: clustering was broken since the switch to AdvancedMarkerElement (marker.getPosition is not a function). The outdated MarkerClusterer v1 is replaced by the official @googlemaps/markerclusterer ^2.6.2. A custom renderer keeps using the cluster images m1.png–m5.png of markerClusterer.imagePath.
• The library is loaded on demand via AssetCollector in partial LoadAssets instead of page-wide page.includeJSFooterlibs, so the FlexForm option also works if clustering is disabled in TypoScript.

Build / Docs

• build.mjs ships both vendor libraries incl. licenses.
• Documentation (Plugins, TypoScript) and ChangeLog (13.2.0) updated.

Tested manually (TYPO3 14.3, ddev)

• Google Maps: ~30 POIs, clusters render with the existing images, click zooms in and splits clusters, no console errors.
• OpenStreetMap: clusters render, no coverage polygon on hover, custom styleSheet via EXT: and fileadmin/ path is loaded.
• Rendered output checked for FlexForm override default/on/off with TypoScript on/off for both providers.

Open

• Unit tests for SettingsHelper::prepareMarkerClusterer().
• Version bump in ext_emconf.php on release.

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