Show waypoint photos from newer p4n2gpx exports

A newer p4n2gpx run (Ruby rewrite, ~/test/mapping/p4n2gpx) started
emitting one plain GPX <link href> per park4night listing photo,
which this app parsed but never displayed. Gpx.waypoint_photos
collects them into a photos property; the waypoint popup renders them
as a row of thumbnails above the description, each opening full-size
in a new tab. Absent on older exports and plain GPS waypoints, so the
row just doesn't render for those.

docs/architecture.adoc

@@ -415,7 +415,8 @@ Like the radar overlay, GPX layers are removed by `map.setStyle()`
 whichever files were checked.
 
 Waypoint popups:: Clicking a waypoint badge opens a MapLibre popup
-with its name, category (if any), description and time. Click/hover handling is
+with its name, category (if any), photos (if any), description and
+time. Click/hover handling is
 registered once per file (via `map.on(type, layerId, handler)`, a
 delegated listener that survives `setStyle()`) rather than on every
 `style.load` re-add, which would otherwise stack duplicate handlers
@@ -442,6 +443,16 @@ before, one paragraph with line breaks. So this only ever adds
 structure on top of recognized lines; it never loses or hides text from
 files that don't have any.
 +
+A newer p4n2gpx export also emits one plain GPX `<link href>` per
+listing photo (not a vendor extension like `waypoint_style`'s
+`<osmand:*>` — `<link>` is base GPX 1.1, any number allowed per
+`<wpt>`/`<trk>`/`<rte>`). `Gpx.waypoint_photos` collects their `href`s
+into a `photos` property; `formatPhotos` renders them as a row of
+64px thumbnails above the description, each linking to the full-size
+image in a new tab. A plain GPS waypoint or an older export has none,
+so the row just doesn't render — same "only ever adds, never assumes"
+approach as the description formatting above.
++
 `Services`/`Activities` get one further step (`formatMetaRow`,
 `formatTags`): the value is park4night's own comma-separated codes
 (`animaux, point_eau, wc_public`), which on their own told a reader