Rebuild weather effects on WebGL2 with new variants and a live demo page

The background weather is now GPU-rendered: one instanced-quad draw call
over a baked sprite atlas plus a fullscreen sky shader (fog, Saharan
haze, aurora, sun rays, storm gloom and lightning flash). The old
Canvas2D renderer stays as weather-2d.js and kicks in automatically
when WebGL2 is missing; weather.js is now a thin controller that owns
the toggle, prefs and the PeteWeather API.

Effect upgrades: shared wind with gust pulses leans the whole scene
together, rain gets depth, splash pops and velocity-aligned streaks,
storms grow procedural branched lightning bolts, snow mixes soft motes
with spinning six-arm crystals, clouds drift in two parallax layers,
clear nights get a moon with maria, twinkling stars and the occasional
shooting star, blossoms and leaves tumble with a faked 3D flip.

New variants: haze (Saharan calima), wind (autumn gusts with streak
lines), hail (bouncing stones with drizzle) and aurora. The /weather
demo page switches variant, intensity and phase in place without a
reload and shows the active renderer plus an FPS meter.

internal/web/static/sw.js

@@ -4,7 +4,7 @@
 //
 // Bump CACHE_VERSION whenever the precached shell assets change; activate()
 // drops every cache that doesn't match the current version.
-var CACHE_VERSION = "v3";
+var CACHE_VERSION = "v4";
 var SHELL_CACHE = "pete-shell-" + CACHE_VERSION;
 var RUNTIME_CACHE = "pete-runtime-" + CACHE_VERSION;
 
@@ -13,6 +13,8 @@ var RUNTIME_CACHE = "pete-runtime-" + CACHE_VERSION;
 var SHELL_ASSETS = [
   "/static/css/output.css",
   "/static/js/prefs.js",
+  "/static/js/weather-2d.js",
+  "/static/js/weather-gl.js",
   "/static/js/weather.js",
   "/static/js/weather-forecast.js",
   "/static/js/search.js",