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>

TODO.md

@@ -27,13 +27,12 @@ no errors), but eyes and fingers haven't confirmed:
 2. Trend arrows appear ~5 minutes after boot (indoor big card, outdoor and
    pressure cells) once the local ring has history. They are local-history
    based and reset on reboot — that is by design, not a bug.
-3. Artefact lifetime: interactions schedule a one-shot full repaint 600ms
-   after the last touch, and every poll repaints too. Artefacts should now
-   visibly self-clean in under a second after touching, and within a poll
-   otherwise. If they *persist* beyond that, the repaint isn't reaching
-   them; if they merely *recur* during interaction, that is the PSRAM
-   bandwidth ceiling (README) and the honest remaining levers are fewer
-   redraws or 120MHz PSRAM on a newer IDF.
+3. Artefact lifetime. Two "repaint to clean up" mitigations were tried and
+   both made it worse -- see README Gotchas. They are gone, the 1Hz restyle
+   that made the artefacts permanent is fixed, and the bounce buffer is up to
+   20 scanlines. If lines still come and go, the remaining honest levers are
+   fewer redraws still, less WiFi interrupt load during rendering, or 120MHz
+   PSRAM on a newer IDF.
 
 ## Done