Attach background effects through one switchable pipeline

- One pipeline, shared by the pre-join preview and the call, attached the
  first time an effect is wanted and never detached after; turning blur
  off switches it to a disabled mode in place. Main dropped the processor
  whenever blur went off, and a pipeline taken off the camera is destroyed
  and re-primed, so each time blur came back one unprocessed frame went
  out. Built once, the priming frame is spent once.
- Still driven by the existing blur setting: nothing a user sees changes.
- Its logic is BackgroundEffects in src/state/, which the provider only
  builds, so the tests drive it without rendering anything.
- Disabled drops the blur radius too. The SDK's disabled mode keeps it,
  and with a radius every frame is still segmented and thrown away; with
  neither a radius nor a picture, frames pass untouched.
- Switches run one at a time and skip any overtaken, so an earlier,
  slower switch can't land after a later choice.
- Builds, rebuilds and teardowns of the pipeline run one at a time
  (OneStepPipeline). The preview's and the call's tracks attach and stop
  it independently, and the SDK's wrapper holds one track's streams: two
  builds at once overwrite each other's, and a stop during a build does
  nothing.
- Every camera track starts with the pipeline on: the room's capture
  defaults follow it, and a camera turned off before the first effect is
  replaced on turning it on, since a stopped track can't take one. One
  attached after publishing lets the room through while it is built.
- BlurBackgroundTransformer becomes BackgroundEffectTransformer: it also
  sits disabled, and its init applies that state as the SDK's would.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/livekit/OneStepPipeline.ts

@@ -0,0 +1,43 @@
+/*
+Copyright 2026 Element Creations Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { BackgroundProcessorWrapper } from "@livekit/track-processors";
+
+type InitOptions = Parameters<BackgroundProcessorWrapper["init"]>[0];
+type DestroyOptions = Parameters<BackgroundProcessorWrapper["destroy"]>[0];
+
+/**
+ * A background pipeline that is built, rebuilt and destroyed one step at a
+ * time. The camera tracks sharing it attach and stop it independently, and
+ * the SDK's wrapper holds a single track's streams: two builds at once
+ * overwrite each other's, and a destroy during a build does nothing.
+ */
+export class OneStepPipeline extends BackgroundProcessorWrapper {
+  private steps: Promise<void> = Promise.resolve();
+
+  public override async init(opts: InitOptions): Promise<void> {
+    return this.inTurn(async () => super.init(opts));
+  }
+
+  // The SDK's restart calls init and destroy, which would wait on itself.
+  public override async restart(opts: InitOptions): Promise<void> {
+    return this.inTurn(async () => {
+      await super.destroy({ willProcessorRestart: true });
+      await super.init(opts);
+    });
+  }
+
+  public override async destroy(options?: DestroyOptions): Promise<void> {
+    return this.inTurn(async () => super.destroy(options));
+  }
+
+  private async inTurn(step: () => Promise<void>): Promise<void> {
+    const turn = this.steps.then(step);
+    this.steps = turn.catch(() => {});
+    return turn;
+  }
+}