feat(moq-video): native Windows Media Foundation capture, drop nokhwa

[kixelated]

Summary

Implements native Windows webcam capture via Media Foundation, replacing the last nokhwa dependency. Frame capture is now native on all three platforms: AVFoundation/ScreenCaptureKit (macOS), V4L2 (Linux), Media Foundation (Windows).

• New capture/mediafoundation.rs drives an IMFSourceReader with the source reader's video processor enabled, so whatever the camera emits (MJPEG / YUY2 / NV12) is coerced to NV12. Each sample is copied to a tightly-packed CPU I420 (new I420::from_nv12, a chroma deinterleave with no color conversion) for the openh264 / NVENC path.
• Devices are enumerated via MFEnumDeviceSources and selected by index or a friendly-name substring (--camera).
• COM + Media Foundation init/teardown is balanced per open/close on the single blocking capture thread; Drop calls IMFMediaSource::Shutdown so the camera releases promptly (LED off) when the last viewer leaves, preserving the on-demand open/close model that the rest of moq-video relies on.
• Removes the nokhwa dep; adds windows = "0.62" (resolves to the 0.62.2 already in the tree via netwatch, so no duplicate version). webcam.rs deleted.
• Docs: DESIGN-native-codecs.md, crate-level docs, and doc/bin/cli.md (which still had stale ffmpeg-era "dshow / h264_videotoolbox" capture wording).

Public API

No public API or wire change. Capture is internal to moq-video; this swaps the Windows backend implementation and removes a dependency. Config, Encoder, Producer, publish_capture, and Error are unchanged. Targets dev, where the rest of the moq-video native-codecs overhaul (#1691, #1704) lives and which the DESIGN doc designates for this work.

Test plan

• cargo clippy -p moq-video clean (host/macOS)
• cargo test -p moq-video — 10/10 pass
• moq-cli --features capture builds (host)
• Full Media Foundation FFI type-checks against x86_64-pc-windows-msvc (verified in a scratch crate; the whole crate can't cross-compile from the dev Mac because openh264's vendored C++ build needs MSVC)
• Real Windows + webcam run — not yet done. Needs to confirm (1) the video processor delivers NV12 for common cameras, (2) IMF2DBuffer::ContiguousCopyTo yields unpadded NV12 so the deinterleave is correct, (3) the open / Shutdown / reopen cycle releases and reacquires the camera cleanly. Tracked in the DESIGN doc's "unverified" section.

Remaining DESIGN cleanup after this: re-introduce VAAPI once cros-codecs builds against libva ≥ 2.23.

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(Written by Claude)