feat(manifold): add dedicated ParticleBackend (no-op transport, gates audio)

Registers a named ParticleBackend in the BackendManager + barrel in place of
the inline particles PassthroughBackend. Transport is a no-op (the flow-field
canvas reads engine outputs directly via rAF); selecting it gates synth audio
(setMuted) like the other non-synth backends and reports a clear ready status.

manifold/src/backends/index.ts

@@ -18,6 +18,7 @@ export { OscBridgeBackend } from './osc-backend';
 export type { OscBackendConfig } from './osc-backend';
 export { NispsOscClient } from './osc-client';
 export { PassthroughBackend } from './passthrough-backend';
+export { ParticleBackend } from './particle-backend';
 export {
   useBackendManager,
 } from './useBackendManager';

manifold/src/backends/manager.ts

@@ -21,6 +21,7 @@ import type { BackendId } from '../dock/output-state';
 import { WebMidiBackend } from './midi-backend';
 import { OscBridgeBackend } from './osc-backend';
 import { PassthroughBackend } from './passthrough-backend';
+import { ParticleBackend } from './particle-backend';
 
 /** The slice of EngineApi the manager depends on (keeps it decoupled/testable). */
 export interface ManagerEngine {
@@ -47,7 +48,7 @@ export class BackendManager {
       ['midi', backends?.midi ?? new WebMidiBackend()],
       ['osc', backends?.osc ?? new OscBridgeBackend()],
       ['synth', backends?.synth ?? new PassthroughBackend('synth', 'Built-in Synth — audio plays in the engine')],
-      ['particles', backends?.particles ?? new PassthroughBackend('particles', 'Particle visualiser')],
+      ['particles', backends?.particles ?? new ParticleBackend()],
       ['cvgate', backends?.cvgate ?? new PassthroughBackend('cvgate', 'CV / gate (via VCV bridge)')],
       ['vcv', backends?.vcv ?? new PassthroughBackend('vcv', 'VCV bridge')],
     ]);

manifold/src/backends/particle-backend.ts

@@ -0,0 +1,35 @@
+/**
+ * ParticleBackend — the output backend for the Particle System mode (the DEFAULT
+ * mode).
+ *
+ * Transport is a NO-OP: the flow-field visualiser (console/flow-field.ts, driven
+ * by ParticleStage) is a SEPARATE consumer of the engine spine — it reads
+ * `engine.getOutputs()` directly in its own requestAnimationFrame loop and maps
+ * the first 20 outputs onto the visual field. So there is nothing for the
+ * BackendManager to "send" here.
+ *
+ * What this backend DOES contribute:
+ *   - It is a non-synth backend, so selecting it makes the BackendManager gate
+ *     audio (`engine.audio.setMuted(true)`) — the synth is silenced while the
+ *     particles are on screen, exactly like the MIDI / OSC modes.
+ *   - It reports a sensible "ready" status for the Outputs panel.
+ *
+ * It extends PassthroughBackend (the shared no-op sink) purely to keep the
+ * empty-`send` / nothing-to-start behaviour in one place; the audio gate lives
+ * in the manager (`setActive`), not here.
+ */
+import { PassthroughBackend } from './passthrough-backend';
+import type { BackendStatus } from './backend';
+
+export class ParticleBackend extends PassthroughBackend {
+  constructor() {
+    super('particles', 'Particle visualiser — outputs drive the flow field (no audio)');
+  }
+
+  override status(): BackendStatus {
+    return {
+      state: 'ready',
+      message: 'Particle visualiser — outputs drive the flow field (no audio)',
+    };
+  }
+}