Name the background effect pieces after background effects

- One name, as review asked: TrackProcessorContext becomes
  BackgroundEffectsContext, with BackgroundEffectsProvider,
  BackgroundEffectsState, useBackgroundEffects,
  useBackgroundEffectsState$ and useSyncBackgroundEffects.
- The state's Behavior is backgroundEffectsState$ wherever it is passed,
  in place of trackProcessorState$, trackerProcessorState$ and
  processorState$.
- Renames only: nothing changes in behaviour.

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

src/state/BackgroundEffects.ts

@@ -15,7 +15,7 @@ import { deepCompare } from "matrix-js-sdk/lib/utils";
 
 import { type Behavior } from "./Behavior";
 import { type ObservableScope } from "./ObservableScope";
-import { type ProcessorState } from "../livekit/TrackProcessorContext";
+import { type BackgroundEffectsState } from "../livekit/BackgroundEffectsContext";
 
 const blurRadius = 15;
 
@@ -30,7 +30,7 @@ export interface BackgroundEffectsOptions {
 
 /** The background effect pipeline, as the camera tracks and the menus see it. */
 export class BackgroundEffects {
-  public readonly state$: Behavior<ProcessorState>;
+  public readonly state$: Behavior<BackgroundEffectsState>;
 
   public constructor(
     scope: ObservableScope,
@@ -38,7 +38,7 @@ export class BackgroundEffects {
   ) {
     this.state$ = scope.behavior(
       blur$.pipe(
-        scan<boolean, ProcessorState>(
+        scan<boolean, BackgroundEffectsState>(
           (previous, wanted) => {
             // Attached the first time an effect is wanted and never detached
             // after, so someone who never turns one on pays for none of it.