Add Block Mosh, Neon City and Flora visualizers

Three new scene shaders to widen the library, now that the main families are
settled. Each is a shader plus a params block, registered in the single
source of truth (scenes/registry.js), so casting, UI, arc and checks pick
them up automatically.

- Block Mosh (glitch): a datamosh. Blocks pull the feedback buffer along
  per-block strokes on a bar-quantized grid so the corruption steps like an
  edit rather than crawling, and spills on onsets. Fills the most iconic
  gap in the glitch family.
- Neon City (structural): a receding skyline with instanced lit windows,
  distinct from the rolling ridgelines and the road grid it sits alongside.
- Flora (organic): an abstract plant of swaying stalks, teardrop petals and
  a bloom corona, all stamped in the track's signature shape.

Also adds HOWTO-visualizers.md, a quick reference for building the next one.

flow-state/README.md

@@ -60,6 +60,8 @@ npm run lint:scenes  # determinism grep + scene schema/shader agreement
 
 ## Adding a scene
 
+Quick walkthrough: `HOWTO-visualizers.md`.
+
 A scene is a shader plus a params block. Everything else — uniform binding, UI
 controls, seeded per-track sampling, arc automation — is derived from the schema.