Rework static overlay as a procedural VHS glitch band

Replaced the sparse-noise-specks static effect with a jagged horizontal
glitch band (plus scattered specks), still procedurally generated with
Pillow -- no source footage involved. Tried deriving this look from a
real, explicitly free-to-use "VHS glitch" stock clip first, but dropped
that approach: even downsampled to 32x32 and thresholded for
transparency, recognizable footage content showed through at any opacity
that read as an actual band rather than noise, and it compressed
noticeably worse than procedural noise. Verified payload stays well
under the 460KB limit against the live station.

README.md

@@ -31,8 +31,9 @@ app's config when adding it to a device.
   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
+  TV-glitch effect (a jagged horizontal static band, plus scattered noise
-  first/last few seconds of a track, as a transition cue.
+  specks) over the album art during the first/last few seconds of a
+  track, as a transition cue.
 
 ## How it works
 
@@ -115,17 +116,25 @@ 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
+- When enabled (Static overlay config), shows a brief TV-glitch 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
+  GIF (`images/static.gif` — a jagged horizontal static band plus
-  background, built with Pillow) composited over the zoomed art via
+  scattered noise specks, over a transparent background, procedurally
-  `render.Stack`, not generated at render time: pixlet's Starlark sandbox
+  generated with Pillow, not sourced from any existing footage)
-  has no per-frame procedural drawing hook, and `animation.Transformation`
+  composited over the zoomed art via `render.Stack`, not generated at
-  only interpolates between fixed keyframes, so there's no way to get
+  render time: pixlet's Starlark sandbox has no per-frame procedural
-  genuine random noise out of it. `render.Image` plays back animated GIFs
+  drawing hook, and `animation.Transformation` only interpolates between
-  natively, looping independently of the zoom's own animation timeline.
+  fixed keyframes, so there's no way to get genuine random noise out of
-  Like the progress bar, this only works correctly on a long-lived server
+  it. `render.Image` plays back animated GIFs natively, looping
+  independently of the zoom's own animation timeline. An earlier version
+  tried deriving this effect from a real "VHS glitch" stock-footage clip
+  (downloaded, downsampled, and thresholded for transparency) instead of
+  generating it — dropped because, even licensed as free-to-use, cropping
+  it down to 32x32 let recognizable footage content show through at any
+  opacity that read as an actual "band" rather than sparse noise, and it
+  compressed noticeably worse than procedural noise does. 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