Add VR demo implementation plan: "Look & Interact" orientation-driven interaction

[Copilot]

Design document for a VR/metaverse demo that maps SpinStep's quaternion primitives to attention-cone-driven interaction — no menus, just natural looking and turning.

docs/vr-demo-design.md

• Capability mapping: Shows how existing primitives (is_within_angle_threshold, batch_quaternion_angle, get_relative_spin, DiscreteOrientationSet.query_within_angle) already implement the core attention-cone mechanics
• Gap analysis: 4 new modules (~400 LOC), 4 utility functions — all compose existing math, no reimplementation
  • AttentionCone — wraps angle-threshold checks with falloff/attenuation
  • SceneEntity / AttentionManager — Node subclass + batch scene queries
  • NPCAttentionAgent — perception cone + slerp-based smooth turning
  • MultiHeadAttention — independent visual/audio/haptic cones
• API design with usage examples for gaze, NPC awareness, spatial audio gain, multi-modal attention
• Unity/Unreal integration: WebSocket bridge protocol (JSON) and native plugin path
• 5-phase delivery plan with file map, LOC estimates, and testing strategy

No source code changes — plan only, per issue requirement. Zero existing files modified; all 46 tests still pass.

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