Improve responsiveness in dense path regions while keeping path discoverability

[napakalas]

In dense areas (for example around spinal pathways), the viewer becomes slow during hover, zoom, and pan.
This is likely not only a viewer-side interaction issue, but also related to the increased number and overlap of paths produced by the map data.

Related issue:

This should be tracked together with flatmap-maker coverage/zoom work:
https://github.com/AnatomicMaps/flatmap-maker/issues/191

Requested behavior:

1. Out-of-zoom paths stay visible but dimmed.
2. Out-of-zoom paths are not interactive (no hover, click, or node selection).
3. Out-of-zoom paths remain searchable.

This will:

• Improve responsiveness in dense path regions.
• Reduce visual/interaction clutter while preserving context.
• Keep paths discoverable via search.