remove: skin_tone_trace node type

- skin_tone is a person attribute (like height), not trace attribute
- Remove build_skin_tone_trace_nodes function
- Remove skin_tone_trace_nodes from TkgResult and API response
- Remove skin_tone_trace from documentation tables

docs_v1.0/API_WORKSPACE/modules/15_tkg.md

@@ -19,7 +19,6 @@ TKG is a time-aligned knowledge graph built from multi-processor outputs (face,
 | `lip_track` | `lip_track_{id}` | Lip movement synced with speech | `speaker_id`, `lip_area_range` |
 | `text_region` | `text_region_{id}` | Spoken text aligned to time | `speaker_id`, `text`, `start_time`, `end_time` |
 | `appearance_trace` | `appearance_{trace_id}` | Human appearance (clothing) over time | `clothing_color`, `upper_cloth`, `lower_cloth` |
-| `skin_tone_trace` | `skin_tone_{trace_id}` | Fitzpatrick skin tone classification | `fitzpatrick_type` (I–VI) |
 | `accessory` | `accessory_{id}` | Detected accessories | `type` (glasses/hat/etc.), `confidence` |
 | `object` | `object_{class}_{id}` | YOLO-detected object | `class`, `confidence`, `frame_count` |
 | `speaker` | `speaker_{speaker_id}` | ASRX speaker segment | `speaker_id`, `segment_count`, `total_duration` |