feat(core): ignoreParentAlpha — render a node at its own alpha while ancestors fade

[chiefcll]

What

Adds an ignoreParentAlpha boolean prop to nodes. When enabled, the node's world alpha is computed from its own alpha only — instead of parent.worldAlpha * alpha — so the node keeps rendering while its parent (and the rest of the subtree) fades.

renderer.createNode({
  // ...
  ignoreParentAlpha: true,
  parent: fadingParent,  // animate fadingParent.alpha freely
});

• The node's own alpha still applies, and can still be animated.
• Descendants of the ignoring node inherit its world alpha as usual.
• Works for text nodes too; surfaces automatically through INode / INodeProps.

Why

A child's effective alpha is always multiplied by its ancestors', so there was no way to keep an overlay (scrim gradient, badge, focus ring) at full strength while its container fades. The earlier workaround — restructuring the overlay as a sibling of the fading container — forces awkward scene-graph layouts and breaks encapsulation in component frameworks.

How

A 3-line branch in the WorldAlpha update plus a getter/setter raising the same update flags as alpha. Render-list invalidation needed no new triggers: toggling the prop flips the node's isRenderable, which already requests a rebuild.

Reviewer notes

• Alpha-0 boundary (deliberate): Stage.buildRenderList culls whole subtrees at worldAlpha === 0, unconditionally. So the prop only has an effect while every ancestor's alpha is above 0 — once an ancestor reaches exactly 0, the ignoring node is culled with the subtree. This keeps the fully-transparent cull free of bookkeeping (an earlier revision tracked a subtree counter to render through alpha-0 ancestors; it was dropped in favor of the simpler cull). Documented in the prop's TSDoc and pinned by the visual test.
• RTT subtrees: no effect inside them — the RTT root's composited quad still fades as one unit. Documented in the prop's TSDoc.
• Inspector overlay: the DOM mirror can't represent escaping ancestor opacity (CSS opacity composes with no escape hatch); debug-cosmetic only, prop passes through untouched.
• Backend-agnostic by construction: the behavior lives in core (worldAlpha + premultiplied colors), so WebGL and Canvas2D agree.

Testing

• Unit tests in CoreNode.test.ts: default multiply, ignore behavior, own world alpha while the parent fades toward 0, premultiplied colors from own alpha, toggle recompute, no-op set short-circuit, and descendant inheritance. Full suite 225/225 green; lint clean.
• Visual regression test examples/tests/alpha-ignore-parent.ts with certified CI snapshot (captured via the Docker runner, manually inspected): faded parent vs ignoring child, near-zero parent (0.05) with a solid ignoring child, own-alpha + descendant inheritance, text nodes, and toggle-back-off.
• Verified in-browser on WebGL and Canvas2D, in dev and prod (legacy transpile — Chrome 38 path) modes.

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