Display feature count on cluster markers

[dopenguin]

Description

Currently, clustered features (core markers) are displayed as a multi-marker; a static SVG showing three stacked pin shapes. There is no indication of how many features are contained in a given cluster.

The cluster marker SVG should be replaced with a new design (see Figma) that displays the number of contained features as text within the SVG.

Current behavior

• The multi-marker is a static SVG without any count indicator (src/core/utils/markers.ts, makeMultiMarker)
• getMarkerStyle only receives a boolean (multi), not the actual feature count
• Memoization is based on MarkerStyle + the multi flag only

Expected behavior

• A new SVG design for cluster markers that includes a <text> element showing the feature count
• The count is dynamically embedded into the SVG

Affected areas

File	What needs to change
src/core/utils/markers.ts	Replace makeMultiMarker with new SVG template including count; extend getMarkerStyle signature to accept count; adjust memoization to account for count
src/core/utils/map/setupMarkers.ts	Pass the actual feature count (from feature.get('features')?.length) to getMarkerStyle at all call sites (layer styling, hover, selection, pointer move)

Out of scope

• Configurability of count text styling (color, font size, etc.)
• Changes to the single marker