Cache parsed GPX/FIT GeoJSON to fix large POI-export loads

A several-thousand-waypoint park4night area export re-parses the
whole file with REXML on every toggle — ~2.4s for a 5k-waypoint/3.6MB
sample, so tens of seconds for a 50k-waypoint one, likely the actual
cause behind a "Could not load" report on a 41.5MB export. MapLibre
already clusters the waypoint layer client-side, so the bottleneck was
never rendering — cache the GeoJSON conversion alongside the source
file (invalidated by mtime) instead of a bigger bbox/tiling rework.

docs/architecture.adoc

@@ -251,7 +251,12 @@ Rendering:: Checking a file fetches `GET /api/gpx/file/<path>`, routed
 by extension to `Maptool::Gpx` or `Maptool::Fit`
 (`lib/maptool/gpx.rb`/`fit.rb`) for parsing, and both convert to the
 same GeoJSON `FeatureCollection` shape so the frontend doesn't need to
-know which format it got. `Gpx` parses the GPX with REXML: `<wpt>`
+know which format it got. `GpxStore#geojson` caches that conversion
+alongside the source file (`<path>.geojson`, invalidated by mtime) —
+REXML parsing a several-thousand-waypoint POI export (a park4night
+area export, say) can take tens of seconds, and without a cache that
+cost repeats on every single toggle rather than just the first one
+after a save. `Gpx` parses the GPX with REXML: `<wpt>`
 waypoints become `Point` features (with `name`, `desc` and `time`
 carried through as properties), each `<trk>` becomes one
 `MultiLineString` feature (one line per `<trkseg>`), and each `<rte>`