* refactor(video_generate): capability-gated dynamic schema — 6 optional args render only when the active provider/model honors them; fleet capability declarations + declaration<->implementation contract tests
* fix(video_gen): H3/Grok/Happy-Horse/Gemini audio is ALWAYS-ON native, not absent — new audio_native family key + audio_always_on capability surfaces as description line (maintainer catch)
* test(video_gen): duration-span test pins the active-model contract — resolved family's real window, short families not inflated, union fallback still spans 30s
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