v0.2.0 — 视频特效：抠像换背景 · 动效文字 · 转场 · 光效

Adds four video effect capability groups, all verified with the end-to-end selftest (26/26).

• video-matte: AI subject matting (RVM ONNX, auto-downloaded) with background replacement, transparent WebM or matte output, plus a chroma-key path for green screen. Measured 81 fps at 568x320 and 17 fps at 720x1280 on CPU.
• video-text-anim: animated titles generated as standard ASS (fade, four-direction slides, typewriter, pop, bounce, karaoke word highlight, lower-third).
• video-join: all 53 xfade transitions enumerated from the bundled ffmpeg, plus stylized presets flash / glitch / whip-pan / soft-zoom / film-burn.
• video-fx: 13 glow, bloom, leak, trail, grain, vignette, sharpen and grading presets, chainable, with external LUT support and before/after comparison frames.

Still out of scope (needs After Effects): motion tracking, 3D camera moves, particle systems, rotoscoping.