T6.1.2: Wire AV1 into call engine (factory + step tables)

- New: factory.rs — create_video_encoder/decoder dispatch by CodecId with
  platform-aware HW→SW fallback. AV1 encoder: SvtAv1Encoder (universal SW).
  AV1 decoder: VideoToolboxAv1Decoder (macOS M3+) → MediaCodecAv1Decoder
  (Android) → Dav1dDecoder (all platforms fallback).
- controller.rs: codec-specific step tables (H.264/H.265/AV1). AV1 ~30%
  lower thresholds than H.264; H.265 ~20% lower. VideoQualityController
  gains codec field with with_codec()/set_codec()/codec() accessors.
- lib.rs: export factory fns and VideoToolboxAv1Decoder
- wzp-client/Cargo.toml: add wzp-video dependency
- 11 new tests (7 factory + 4 controller); 77→88 wzp-video tests; fmt +
  clippy clean; all workspace tests pass

Cargo.lock

@@ -7811,6 +7811,7 @@ dependencies = [
  "wzp-proto",
  "wzp-relay",
  "wzp-transport",
+ "wzp-video",
 ]
 
 [[package]]