Remove the Snow visual preset

Snow is no longer offered anywhere a style can be chosen: the Visual
Presets tray, keyboard shortcut 7, the Cockpit vision carousel, and the
set_visual_style voice action. The shader and its labels go with it;
weather snowfall and the SNOW weather condition are unaffected.

Share links keep working. Snow's enum code (6) stays reserved, so a
compact or long-form link that names it opens on Normal and drops the
Snow shader parameters without reporting them as unknown. A Scene
Director shot saved with Snow plays on Normal too. Snow's frozen share
defaults matched Normal's apart from those parameters, so no other
setting in an old link changes meaning.

The legacy voice-schema pins restore Snow before hashing, so the
unchanged pins still prove nothing else moved; the complete Realtime
payload digest is re-derived.

docs/PERFORMANCE.md

@@ -97,7 +97,8 @@ operational sample contained 3,909 observations and two selected labels, but its
 live aircraft and traffic rows were empty, so it remains a limited stress case.
 
 Snow, Noir, dense detection, and text-heavy combined layers are the clearest
-controlled comparison points for later optimization work.
+controlled comparison points for later optimization work. Snow has since been
+retired as a visual preset; its row records the measurement as taken.
 
 ## Keyed live sources