feat: Add V3 background segmentation video frame processor

  Implement a new CDN-based background segmentation processor that supports
  blur (low/medium/high), image replacement, and color replacement effects
  with two model types (selfie_general and selfie_multiclass).

  Core:
  - BackgroundSegmentationVideoFrameProcessor with CDN asset loading
  - BackgroundSegmentationAssetLoader with request deduplication
  - BackgroundSegmentationCompatibilityChecker (WebGL2, WASM, Workers)
  - BackgroundSegmentationMetrics observer for StatsCollector integration
  - Dynamic filter switching via setConfig/setModelType without recreation

  Observability:
  - Video processor metrics (frame rate, per-processor latency) via StatsCollector
  - Event publishing for filter lifecycle (backgroundFilterStarted, backgroundFilterConfigSelected, backgroundFilterFailed)
  - Extended VideoFXEventAttributes with V3-specific fields

  Demo:
  - Replace legacy MediaPipe/TensorFlow-based filters with V3 segmentation
  - Add blur strength options (low/medium/high) for both model types
  - Fix iOS crash on rapid filter switching (concurrency guard)
  - Fix filter re-application after offline/online transition
  - Remove @tensorflow-models/body-segmentation dependency

  Testing:
  - Add unit tests for BackgroundSegmentation (processor, asset loader, compatibility checker, metrics)
  - Add barrel export coverage for named re-exports from index.ts
  - Export BackgroundSegmentationCompatibilityResult, BackgroundSegmentationMetricsObserver, BackgroundSegmentationMetricReport from index.ts for documentation
  - Update VideoProcessingTest to use V3 background segmentation filters (replaces V2 VideoFx tests)
  - Add VideoTestSteps utility methods for blur, image replacement, and color replacement with model parameter
  - Add fake video stream (output.y4m) for consistent pixel-based filter verification
  - Add blue pixel percentage check and pixel diff verification for filter validation

Author: harshhv-dev

CHANGELOG.md

@@ -5,6 +5,29 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+  
+### Added
+
+- Added `BackgroundSegmentationVideoFrameProcessor` with CDN asset loading, request deduplication, and browser compatibility checking (WebGL2, WASM, Workers)
+- Added configurable blur strength (low/medium/high) and support for `selfie_general` and `selfie_multiclass` model types
+- Added `BackgroundSegmentationMetrics` observer for StatsCollector integration
+- Added dynamic filter switching via `setConfig`/`setModelType` without processor recreation
+- Added video processor metrics collection (frame rate, per-processor latency) via StatsCollector
+- Added event publishing for filter lifecycle (`backgroundFilterStarted`, `backgroundFilterConfigSelected`, `backgroundFilterFailed`)
+- Added V3-specific fields to `VideoFXEventAttributes`
+
+### Removed
+
+- Removed `MediaPipeBodySegmentationProcessor` and `@tensorflow-models/body-segmentation` dependency from demo
+
+### Changed
+
+### Fixed
+
+- Fixed iOS page crash when rapidly switching background filters
+- Fixed video filter not re-applying after offline/online transition
+
 ## [3.32.0] - 2026-05-14
 
 ### Added