Maps · Layers: routes, GeoJSON, heat and clusters

MapView takes layers over its tiles and under its pins. RouteLine runs
along places the short way across the date line. read_geojson reads RFC
7946 to features, strictly: a position past the globe, a line of one
position or a ring that does not close fails with its reason, and a
feature placed nowhere keeps no geometry. GeoJsonLayer washes areas as
one path per polygon, so holes stay open, outlines them, strokes lines
and dots points. GeoHeatmap spreads weighted places in a quartic kernel
over small cells, scaled to the densest spot in view. Lines and areas
unwrap across the date line and repeat on every world in view.
cluster_markers gathers pins by squares of the world, so a pan keeps
each group; a cluster shows its count, and its press shows its members
whole, never farther out. A zoom that may regroup them hands a focused
pin's or cluster's focus to the map.

tasks/ch31-43.md

@@ -139,8 +139,8 @@
 - [x] SystemMonitor → `devtools::ResourceMonitor`
 - [x] ProcessTable → `terminal::ProcessList`
 - [x] NetworkGraph (throughput) → `charts::RealtimeChart`
-- [ ] MapView / GeoHeatmap → `maps::MapView`, `maps::GeoHeatmap` (T41)
-- [ ] WorldMap → `maps::WorldMap` (T41)
+- [x] MapView / GeoHeatmap → `maps::MapView`, `maps::GeoHeatmap` (T41a, T41b)
+- [ ] WorldMap → `maps::WorldMap` (T41c)
 - [x] RefreshIntervalSelector
 - [x] DashboardFilterBar
 
@@ -298,10 +298,10 @@
 
 ## T41b Maps · Layers
 
-- [ ] MapCluster
-- [ ] RouteLine
-- [ ] GeoJSON Layer
-- [ ] GeoHeatmap (from T34)
+- [x] MapCluster → `MapView::cluster_markers`: pins gathered by squares of the world, so a pan keeps each group; a cluster's press shows its members whole
+- [x] RouteLine → `maps::RouteLine`, a layer that crosses the date line the short way
+- [x] GeoJSON Layer → `maps::read_geojson` reads RFC 7946 to features; `maps::GeoJsonLayer` washes areas with their holes open, strokes lines, dots points
+- [x] GeoHeatmap (from T34) → `maps::GeoHeatmap`, weighted places as heat scaled to the densest spot in view
 
 ## T41c Maps · World