ui(map): contrast pin labels against active basemap

Marker (pin) labels previously inherited the app's default text color,
which reads white and disappears on light basemaps. Set each label's
text color in applyFrame() against the active style: black by default
(light basemaps osm-bright/positron/toner) and white only on the one
dark basemap (dark-matter). Re-evaluated every frame so a style switch
re-colors visible labels on the next applied frame.

Add a contract test pinning the dark-basemap detection and the
black-by-default / white-on-dark ternary.

lib/tdeck_ui/UI/LXMF/MapScreen.cpp

@@ -100,6 +100,13 @@ const char* styleLabel(const char* style_id) {
     return "Style";
 }
 
+// The allowlist is otherwise light basemaps (osm-bright, positron, toner).
+// Marker labels sit directly on those tiles with no backing plate, so black
+// reads on light backgrounds and white only on the one dark basemap.
+bool isDarkBasemap(const char* style_id) {
+    return style_id != nullptr && std::strcmp(style_id, "dark-matter") == 0;
+}
+
 }  // namespace
 
 static_assert(sizeof(lv_color_t) == 2U,
@@ -728,6 +735,13 @@ void MapScreen::applyFrame() {
             setPlaceholder(index);
         }
     }
+    // Marker labels float directly over the basemap tiles (no backing plate),
+    // so their ink must contrast the active style: black on light basemaps,
+    // white on the one dark basemap. Re-evaluated every frame so a style
+    // switch re-colors any visible labels on the next applied frame.
+    const lv_color_t marker_label_color =
+        isDarkBasemap(style_selector_.activeId())
+            ? lv_color_white() : lv_color_black();
     for (std::size_t index = 0; index < MARKER_COUNT; ++index) {
         if (index >= frame.marker_count) {
             lv_obj_add_flag(approximation_halos_[index], LV_OBJ_FLAG_HIDDEN);
@@ -764,6 +778,7 @@ void MapScreen::applyFrame() {
                           marker.peer.bytes[Telemetry::PEER_ID_SIZE - 1U]);
         }
         lv_label_set_text(marker_labels_[index], label);
+        lv_obj_set_style_text_color(marker_labels_[index], marker_label_color, 0);
         lv_obj_set_pos(marker_labels_[index], x + 6, y - 7);
         lv_obj_clear_flag(marker_labels_[index], LV_OBJ_FLAG_HIDDEN);
     }