Add an optional TV-static overlay during track start/end

Shows a brief static effect (sparse noise specks, bundled as a small
animated GIF) over the album art during the first/last 5 seconds of a
track, as a transition cue. Implemented via render.Stack compositing a
bundled images/static.gif over the zoomed art -- pixlet has no per-frame
procedural drawing hook, so the noise is pre-baked with Pillow rather
than generated at render time; render.Image plays animated GIFs back
natively, independent of the zoom's own animation timeline.

Tuned the GIF's frame count/density/flicker speed down after an initial
version pushed total payload within a few percent of the 460KB device
limit on some album art -- more, faster-changing frames give WebP less
opportunity to dedupe near-identical frames, which compounds with the
zoom animation's own dedupe needs. Final version adds ~50KB regardless of
art complexity, verified via a pinned-cover test harness for reliable
before/after comparison (the live station's currently-playing track kept
changing between test runs otherwise).

Added a "Static overlay" On/Off schema toggle (default On) per request.

README.md

@@ -30,6 +30,9 @@ app's config when adding it to a device.
   *every* refresh, not just when the track changes, so a long track gets
   to show off different sections of its art over time instead of being
   stuck on one spot for its whole runtime.
+- **Static overlay** — On (default) or Off. When on, shows a brief
+  TV-static effect (sparse noise specks) over the album art during the
+  first/last few seconds of a track, as a transition cue.
 
 ## How it works
 
@@ -112,6 +115,25 @@ app's config when adding it to a device.
   its full module list) — so there's no way to pre-bake or crop-at-render-
   time from a source we don't have until the request hits the live
   station.
+- When enabled (Static overlay config), shows a brief TV-static effect
+  over the album art during the first/last `STATIC_OVERLAY_SECONDS` (5s)
+  of a track, as a transition cue. Implemented as a small bundled animated
+  GIF (`images/static.gif` — sparse noise specks over a transparent
+  background, built with Pillow) composited over the zoomed art via
+  `render.Stack`, not generated at render time: pixlet's Starlark sandbox
+  has no per-frame procedural drawing hook, and `animation.Transformation`
+  only interpolates between fixed keyframes, so there's no way to get
+  genuine random noise out of it. `render.Image` plays back animated GIFs
+  natively, looping independently of the zoom's own animation timeline.
+  Like the progress bar, this only works correctly on a long-lived server
+  with a persistent cache — a one-shot `pixlet render` CLI call always
+  sees `elapsed=0`, so it'll always show the "starting" static, never the
+  "ending" one. Tuned the GIF's frame count/density/flicker speed down
+  (fewer, sparser, slower-changing frames) after an initial version came
+  within a few percent of the 460KB device limit on some album art — more
+  distinct frames changing more often give WebP less opportunity to
+  dedupe near-identical frames, and that cost compounds with the zoom
+  animation already using up dedupe headroom of its own.
 - Shows the current show name (`context.activeShow.name`) and host name
   (`dj.name`) below the artist, separated by a thin divider line, and the
   local time/temperature (`context.clock`/`context.weather`) below that —