Add BVH motion export #63

Summary

Add a BVH export path so Posecode movements can be imported into Blender and other animation tools.

This was requested by an early user who wants to export authored movements for use outside the browser renderer.

Desired workflow

• author or generate a movement in Posecode
• preview and validate it in the existing renderer
• export the movement as a .bvh file
• import the result into Blender or another BVH-compatible tool

Design questions

• export raw authored joint motion or the final solved motion after grounding, IK, turn, and travel
• define a stable skeleton hierarchy, coordinate system, scale, and joint channel order
• support a configurable sample rate such as 30 or 60 fps
• decide whether export belongs in posecode-render, a dedicated package, the CLI, the playground, or a combination

The final solved motion is likely the more useful default because it preserves the movement users actually see in the renderer.

Acceptance criteria

• a canonical Posecode movement exports as a valid BVH file
• the file imports successfully into Blender
• duration, frame rate, root motion, and joint rotations are preserved
• coordinate system and scale are documented
• at least one automated export test covers the BVH hierarchy and frame data
• the playground or documented API provides a clear way to download the result