[dajiaohuang]

Changes

• Document that video_compose.render does not apply edit_decisions.audio or metadata.grade_profile automatically, and that audio_path replaces the rendered video's audio track.
• Describe explicit audio_mixer inputs for planned audio, including the segmented_music path and its input limitations.
• Replace unsupported grade names with the built-in color_grade profile IDs and correct the body render order.

This is a partial follow-up to #575. The edit_decisions.end_tag schema mismatch was already addressed by merged PR #813; this change does not claim to close the remaining issue.

Validation

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