fix(ui): render unscored pose keypoints

[alloutflo]

Summary

• treat a completely zero-confidence keypoint set as unscored at the rendering boundary
• use a neutral display-only confidence copy across all four renderer modes
• preserve source data and continue rejecting zeroes in mixed scored/unscored sets
• add the canvas regression to the blocking PR UI job

This makes the shipped signal-derived/no-model coordinates visible without presenting the fallback value as sensing confidence.

Validation

• node --check ui/utils/pose-renderer.js
• PR UI command — 29/29 passed, including 3 renderer regressions
• git diff --check upstream/main...HEAD

The renderer boundary is covered with a canvas stub; no Docker, hardware, or pixel-level browser claim is made.

Closes #1525