Add an Alt+click debug inspector, NPMap4-infobox style

NPMap4 has infoboxControl: a generic "click anything, show every raw
property" debug view, kept separate from a dataset's own curated popup.
Not portable code (Leaflet), but the concept is: Alt+click anywhere now
opens a popup listing every rendered feature at that point via
map.queryRenderedFeatures — base map OSM tags included, not just this
app's own GPX/FIT/radar layers — with a raw key/value table per feature.

No dedicated UI for it on purpose: a keyboard-modified click needs no
button, toggle state, or cursor change of its own, and it's fine for it to
coexist with a layer's own click handler firing on the same click (e.g. a
GPX waypoint's normal popup) since this is a power-user/debugging
affordance, not a replacement for those. Feature properties can come from
an uploaded GPX/FIT file (untrusted input) via this app's own layers, so
every value is escaped the same way gpx.js's popups already are.

docs/architecture.adoc

@@ -154,6 +154,24 @@ manages in the page's own sidebar — both call the same
 `LayerToggleControl#toggle()`, so there's one source of truth for the
 panel's open/closed state either way.
 
+=== Debug inspector (Alt+click)
+
+Alt+click anywhere on the map opens a popup listing every rendered
+feature at that point — across all layers, not just this app's own GPX/
+FIT/radar layers — with its raw properties (`public/js/map.js`, a plain
+`map.on("click", ...)` handler gated on `event.originalEvent.altKey`,
+calling `map.queryRenderedFeatures(event.point)`). Inspired by NPMap4's
+`infoboxControl`, a similar generic "show me everything here" debug
+view kept separate from the curated popups those same features already
+have. Deliberately no dedicated UI for it — a keyboard-modified click
+needs no button, toggle state, or cursor change of its own, and it
+coexists fine with layer-specific click handlers (e.g. a GPX waypoint's
+normal popup) firing on the same click, since this is a power-user/
+debugging affordance, not a replacement for those. Feature properties
+can come straight from an uploaded GPX/FIT file (untrusted input) via
+this app's own layers, not just the base map, so every key/value is
+HTML-escaped the same way `gpx.js`'s popups already are.
+
 == Weather radar overlay
 
 `public/js/radar.js` adds a "Radar" map control that overlays recent

docs/nps-resources.adoc

@@ -266,8 +266,16 @@ plugins (several still at `0.0.1-x`, pre-1.0), a few of which (`overview`,
 `carto`, `wms`) aren't even merged to `main` yet — see above. v5 is the
 direction NPS is migrating to, not (yet) a feature-for-feature
 replacement; expect this gap to close over time, not to stay this way.
-Two things worth noting from v4:
+Three things worth noting from v4:
 
+* *`infoboxControl`* — example:
+  https://www.nps.gov/lib/npmap.js/4.2.1/examples/basic/infobox.html. A
+  generic "click anything, show every raw property in a table" debug
+  view, kept separate from a dataset's own curated popup. *Adopted* — not
+  the code (Leaflet, not portable) but the concept: this app now has the
+  same thing as an Alt+click handler (see `docs/architecture.adoc`'s
+  "Debug inspector" section), showing `queryRenderedFeatures` results —
+  base map OSM tags included, not just this app's own layers.
 * *Geocoder control* — example:
   https://www.nps.gov/lib/npmap.js/4.2.1/examples/controls/geocoder-control.html.
   Config is `geocoderControl: { searchPlaces: true }`: a search box that