[WIP] Implement renderer improvements for edge cases and precision

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

Original prompt

Inspect main/open PRs; implement substantive compatible renderer improvements. Focus clipping/near-plane edge cases, winding/degenerate triangles, depth precision, perspective-correct interpolation, shadow sampling bounds and deterministic small render fixtures. Preserve pure-JS CPU/no-WebGL/no-dependency purpose. Improve resize/input/pause/accessibility and diagnostic clarity. Run baseline/final numerical/render tests and browser smoke, verify docs/CI. No copied assets, secrets, external code/data uploads, paid services, license changes, deployment or auto-merge. Dedicated PR with exact results/tolerances and limitations, no loosened tests or fabricated performance.