perf(l2): no redraws while nothing changes

Found by counting flushes on idle screens in the sim:
- The compose field's cursor blinked (a redraw every 400 ms) as soon as
  a conversation opened, the keyboard hidden: lv_keyboard_set_textarea()
  focuses the field, and LVGL only stops a blink when the cursor moves.
  fieldFocus() sets the state and nudges the blink; also used for the
  WiFi and channel fields, which left theirs running.
- lv_obj_align_to() restyles on every call, so the status bar's icons
  redrew every second; placeLeftOf() moves them only when they move.
- Text colours set every second (nav bar, GPS status, Diag errors) go
  through setTextColor(), which skips an unchanged colour.
- A LV_LABEL_LONG_DOT label writes its "..." into its own text, so
  setText() never matched and rewrote it every refresh (the nav bar's
  hint); labelShows() reads past the dots.
- The map area, the area-outline, trail and vector-label layers were
  invalidated on every map refresh (every 3 s, a full-map redraw); now
  only when the view or what they draw changed.

Idle map: 17 -> 1 flush in 5 s; list and settings screens: 0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

examples/companion_radio/ui-lvgl/HomeScreen.h

@@ -721,8 +721,7 @@ void UITask::homeMapLayout() {
   int shown = 0;
   char sl[16];
   double lat = atan(sinh(M_PI * (1 - 2 * s_cy / (double)n))) * 180.0 / M_PI;
-  s_grid = { left, top, mapview::gridStep(s_z, lat, _prefs && _prefs->units_imperial, 48, sl, sizeof(sl)) };
-  lv_obj_invalidate(s_area);
+  mapview::setGrid(s_area, s_grid, left, top, mapview::gridStep(s_z, lat, _prefs && _prefs->units_imperial, 48, sl, sizeof(sl)));
   bool tiled = true;
   for (int j = 0; j < ROWS; j++) {
     for (int i = 0; i < COLS; i++) {