perf(l2): map tiles decode on the other core

- TileCache: request() hands z/x/y to a task on core 0, which renders into
  a spare buffer; poll() swaps it into the LRU slot (stale results dropped
  after drop()/invalidate()); the sim runs the job at once
- MapScreen / Home map place finished tiles as they arrive; the longest
  UI stall with the map opening goes from ~340 to ~170 ms
- LVGL allocates from the PSRAM heap (lv_mem_psram.h), safe from both
  tasks; LVGL's own pool is gone
- lv_conf: notes why LV_OS_FREERTOS with two draw units stays off
  (measured slower: drawing is bound by PSRAM bandwidth)
- UI_PERF_TEST: the map (tile read/decode, pan, redraw), build time and
  the longest loop pass per screen

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

examples/companion_radio/ui-lvgl/UITask.h

@@ -313,6 +313,9 @@ private:
   void buildHomeMap(lv_obj_t* box);
   void homeFieldText(uint8_t field, char* buf, int n);
   bool homeMapFit();
+#ifdef UI_PERF_TEST
+  void perfMap(bool reset = false);
+#endif
   void homeMapLayout();
   void homeMapLoop();
   void refreshHome();