fix: freeze the scope when the transport is stopped

FL keeps calling process() with silence while stopped/paused, so the scope
scrolled silence instead of holding. Gate the plot's bucket advance on
context.transport().playing, so it freezes on stop/pause and resumes on play.
Bar meters still fall to silence as before. Update README thread-safety notes
for the ScopeRing feed and transport gating.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -256,9 +256,15 @@ NIH-plug provides `Smoother` — use it for all gain/threshold params to avoid z
  
 ### Thread safety
 Params are atomics. The editor and audio thread communicate only through params and a shared
-`Arc<Meters>` of **lock-free atomics** (`meters.rs`) — never a mutex on the audio path. `process()`
-publishes one value per meter per block (gated on the editor being open); the editor reads them
-each frame. Never pass DSP state to the UI directly.
+`Arc<Meters>` (`meters.rs`) — never a mutex on the audio path. Two lock-free feeds, both gated on
+the editor being open:
+- **Bar meters** — decayed atomic scalars, one store per block; the editor reads them each frame.
+- **Scrolling plot** — a single-producer/single-consumer `ScopeRing` of buckets clocked at
+  ~200 Hz, so the plot's horizontal resolution is decoupled from the ~60 fps repaint. The editor
+  drains all new buckets each frame. The scope is **transport-gated** (advances only while playing)
+  so it freezes rather than scrolling silence when the host is stopped/paused.
+
+Never pass DSP state to the UI directly.
  
 ### VST3 licensing
 You must accept Steinberg's VST3 SDK licence before distributing VST3 binaries.