Swap the pin marker for a rounded-square badge

The National Park Service frames its own map symbols (the icons already
vendored in for POI categories) in a rounded square, not a pin — pairing a
pin shape with NPS-style pictograms looked like two mismatched icon
conventions. Switched the marker to a rounded square to match.

This also simplifies the placement code: a pin has to anchor at its tip, so
the category glyph on top needed an offset computed from the pin/glyph
geometry to land in the pin's head. A square has no tip — both the badge and
the glyph layer just center directly on the coordinate with no offset math
at all.

docs/architecture.adoc

@@ -219,38 +219,37 @@ the same local element name "icon" once REXML drops namespace
 prefixes, so this specifically matches on namespace *URI*
 (`https://osmand.net`), not local name, to avoid picking whichever
 happens to come last in the file). Individual waypoints render as a
-pin (`public/js/icons.js`'s `MAP_PIN_ICON`, the same path as the
-existing `WAYPOINT_ICON` outline, filled instead of stroked) rather
+rounded-square badge (`public/js/icons.js`'s `MAP_MARKER_ICON`) rather
 than a plain circle — a flat colored dot turned out to look too much
 like a base map style's own small circular POI icons, blending in
-instead of reading as "your data"; a pin is a shape convention no base
-map uses for its own points. It's colored via
-`["coalesce", ["get", "color"], TRACK_COLOR]` (falling back to the
+instead of reading as "your data". A rounded square rather than a
+pin/teardrop: it matches how the National Park Service itself frames
+these exact category symbols (below) on its own maps, so the two look
+native together instead of like mismatched icon styles. It's colored
+via `["coalesce", ["get", "color"], TRACK_COLOR]` (falling back to the
 plain track color for a waypoint with no category, e.g. a hand-placed
-GNSS pin) and anchored at its tip (`icon-anchor: "bottom"`, the
-standard map-pin convention — the tip, not the shape's center, lands on
-the actual coordinate). A small curated icon set
-(`public/js/icons.js`'s `POI_ICONS`, covering the categories p4n2gpx
-actually uses, not an exhaustive OsmAnd icon set) draws a white glyph
-on top of the pin's head, in a symbol layer offset up from the shared
-tip anchor by a size computed from the pin and glyph geometry
-(`POI_ICON_OFFSET_Y` — both layers anchor to the same raster canvas
-edge, so the offset is "how far above that shared anchor is the pin's
-head-center, minus how far above it the glyph's own visual center
-already sits at its own icon-size"), filtered to just the known keys
-(anything else keeps the plain pin with no glyph, rather than a
+GNSS pin) and — unlike a pin, which has to anchor at its tip — centered
+directly on the coordinate with plain default anchoring, which also
+means the category-glyph layer on top needs no offset math: both
+layers just share the same center anchor at `[0, 0]`. A small curated
+icon set (`public/js/icons.js`'s `POI_ICONS`, covering the categories
+p4n2gpx actually uses, not an exhaustive OsmAnd icon set) draws a white
+glyph on top of the badge for any waypoint whose `icon` matches a known
+key (filtered out entirely for anything else, so an unrecognized icon
+key just falls back to a plain colored badge rather than a
 broken/missing image warning). Six of the seven glyphs are the
 National Park Service's own map symbols
 (`github.com/nationalparkservice/symbol-library`, BSD-licensed,
 originally forked from Mapbox's Maki project) — a professional,
 purpose-built icon set beats hand-drawing "fuel pump"/"picnic table"
 glyphs from scratch; `forest` has no NPS equivalent (that set covers
-facilities, not land cover) and stays hand-drawn. Both the pin and the
-category glyphs are rasterized once per style load (`_registerPoiIcons`,
-an SVG → `<img>` → `<canvas>` → `ImageData` round trip, since these
-need to be real raster images for `map.addImage`) and registered as SDF
-images — a single alpha shape MapLibre tints at render time via
-`icon-color` rather than a fixed-color image. The points source itself
+facilities, not land cover) and stays hand-drawn. Both the badge and
+the category glyphs are rasterized once per style load
+(`_registerPoiIcons`, an SVG → `<img>` → `<canvas>` → `ImageData` round
+trip, since these need to be real raster images for `map.addImage`) and
+registered as SDF images — a single alpha shape MapLibre tints at
+render time via `icon-color` rather than a fixed-color image. The
+points source itself
 is created with
 `cluster: true`; nearby points collapse into a stepped-radius,
 sequential-blue circle sized by `point_count` below `clusterMaxZoom`,