v0.3.0 — engine v4 opt-in

Opt-in engine v4 (engine: 'v4'). Default remains V3.

• Handshake engineChannel; V4 downloads engine-v4.bundle.js (engine 4.0.0)
• Catalog: squat, shoulder_roll, shoulder_deep_breath, chair_forward_fold
• V4-only ids on V3 throw Exercise 'x' requires engine: 'v4'
• Jumps stay V3-only; V4 does not emit recommendations
• Docs: https://docs.posetracker.com/reference/engine-v4