Guard paint layer RGB promotion

[Mistral-Yu]

What changed

• Make Brush detail children inherit the parent paint layer and retire birth-time one-layer promotion.
• Remove the obsolete Contribution-confirmed detail promotion checkbox.
• Keep the training layer-order RGB guard internally enabled by default, repairing stable source-footprint RGB before forward layer motion and deferring high-variation edges.
• Update the public README to describe the resulting behavior.

Why

Periodic RGB exposure was traced primarily to growth events rather than contribution compaction. Immediate child promotion could expose stale colors, while the shared contribution-aware layer path already provides the appropriate later movement point.

Validation

• Related Brush, layer-order, growth, and compaction contract tests passed.
• node --check web/app.js
• node verify-release.mjs
• Pages artifact build and node verify-release.mjs _site
• node scripts/pages_artifact_tests.mjs _site
• 3000-iteration Mac Chrome WebGPU checks completed with the RGB guard enabled and no browser warnings/errors.