Refactor skeleton rendering API

Reworks `display/skeleton.py`. It replaces the old `Skeleton` runtime/model loading stack with immutable skeleton definition/resolution dataclasses, adds validated metadata-to-definition construction, and introduces a standalone `draw_skeleton(...)` path that supports both single and multi-individual poses. Rendering now returns a structured `SkeletonRenderResult` with clearer status codes (including no-pose, invalid shape, keypoint mismatch, and gradient color mismatch) and tracks drawn edge count.

Author: C-Achard