Stop the constant repaint that made the left-edge artefacts permanent

The artefacts went from occasional to continuous, and the cause was my own
mitigation for them.

lv_obj_set_local_style_prop() does NOT skip identical values -- it calls
lv_obj_refresh_style() unconditionally, which invalidates the object. A
comment in ui_set_stale() claimed the opposite and that assumption spread.
ui_update_thermostat() is called once a second by the control task and was
writing seven style properties every time, so the panel was redrawing
something forever, and glitched lines were being regenerated as fast as
anything could clear them.

Two full-screen invalidates made it worse still: one after every HA poll,
one 600ms after every touch. The premise was that LVGL never reinvalidates
pixels it does not know are wrong, so repainting would clean them up. But
the README already had the answer -- the corruption scales with how much is
written, because LVGL's PSRAM writes compete with the panel's DMA reads out
of the same PSRAM. A full repaint is 768KB of exactly the contention that
causes the problem. Both are removed.

What is left is the thing that actually helps: write less.

  - ui_update_thermostat() returns without taking the lock when nothing
    moved, and otherwise touches only the widgets whose values changed.
  - ui_set_stale() is guarded on the previous value.
  - Condition and trend icons only get a colour write when the glyph itself
    changed.

Also raises the bounce buffer from 16 to 20 scanlines. The comment saying
20 "fails to allocate once WiFi is up" was true when display_init() ran
after wifi_init_sta(); it does not any more. display_alloc() takes the
buffers from a virgin heap with ~200KB free, so the extra 12.8KB is
affordable, and each larger chunk buys the refill ISR more time to fetch the
next one before the LCD needs it. That attacks the mechanism rather than the
symptom. ~24KB internal heap remains once WiFi is up; the "Free internal
heap" log line is the thing to watch if it needs to come back down.

Both gotchas are now in the README so the next person does not reach for a
repaint again.

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

main/drivers/display/display_rgb.c

@@ -80,9 +80,22 @@ static const char *TAG = "display_rgb";
  * displays whatever was left in the buffer, showing up as short stale lines
  * at the left edge where each chunk begins.
  */
-/* 16 is the practical ceiling: two buffers at 20 lines need 64KB of internal
- * RAM and the allocation fails once WiFi is up. */
-#define BOUNCE_BUFFER_LINES 16
+/*
+ * 20 lines: two buffers at 800px x 2 bytes = 64KB of internal RAM.
+ *
+ * This used to be 16 because 20 "fails to allocate once WiFi is up" -- true
+ * when display_init() ran after wifi_init_sta(). It no longer does: the
+ * two-phase split means display_alloc() takes these buffers from a virgin
+ * heap before WiFi has touched it (~200KB free at that point), so the extra
+ * 12.8KB is affordable now.
+ *
+ * Bigger directly attacks the artefact mechanism: each chunk buys the refill
+ * ISR that much more time to fetch the next one out of PSRAM before the LCD
+ * needs it. Going further eats the internal RAM WiFi wants at runtime --
+ * watch the "Free internal heap" line, and if it drops near single digits,
+ * come back down.
+ */
+#define BOUNCE_BUFFER_LINES 20
 
 static esp_lcd_panel_handle_t panel = NULL;
 static lv_display_t *disp = NULL;