[codex] Add CI and improve RGBDE viewer responsiveness

[amariichi]

Summary

• add GitHub Actions CI with Python tests plus JavaScript syntax and lint checks
• add a local performance harness, deterministic fixtures, and low-risk RGBDE processing optimizations
• move desktop rendering to on-demand scheduling, shift RGBDE decode/preprocess work into a worker-capable path, and move display-time depth shaping to the GPU while preserving CPU export parity
• improve SBS stereo rendering by switching to off-axis stereo with a stronger default separation

Why

This repository did not yet have PR-time CI guardrails, and several viewer hot paths were still doing unnecessary main-thread and CPU-side work. The stereo path also tended to produce weak SBS differences unless depth magnification was pushed high enough to distort facial geometry.

Impact

• PRs can now be gated on python-tests and js-checks
• RGBDE load and desktop interaction should feel more responsive, especially while idle or during decode/preprocess
• local developers can use /perf-harness.html to compare frontend and backend timing
• SBS stereo should produce clearer left/right disparity without forcing aggressive depth magnification

Validation

• npm run check:js
• npm run lint
• .venv/bin/python -m unittest discover -s tests -p 'test_*.py'
• browser checks for /, /perf-harness.html, RGBDE fixture loading, worker/main-thread benchmark rows, and SBS mode switching
• CPU evaluator parity check for export/display depth shaping (maxDiff: 0)