fix(components): apply the css cascade before painters read style

[LeadcodeDev]

Severity Medium, category correctness. Location: crates/rustmotion-components/src/legacy_dispatch.rs:68

Impact

box_builder.rs:496 calls rustmotion_core::css::cascade::inherit_from(parent_css, &mut css) and paint_pass.rs:479 passes &node.css into dispatch. LegacyPaintDispatcher (the only real PaintDispatcher; the other is NoopDispatcher) binds it to _css and drops it, and Painter::paint_content (painter.rs:60-66) has no CssStyle parameter at all — so a painter structurally cannot see the cascaded style. Every painter resolves typography from its own declared struct field instead (text.rs:523 self.style.color_str_or("#FFFFFF")). All twelve properties inherit_from propagates (color, font-family, font-size, font-weight, font-style, line-height, letter-spacing, text-align, white-space, overflow-wrap, visibility, text-decoration) are paint-only: taffy_bridge.rs reads none of them (its own header says "Paint properties (color, background, transform...)" are excluded), paint_pass.rs contains zero references to visibility/text_decoration/text_align/font_family/white_space, and component_intrinsic (box_builder.rs:639) builds measurers from &child.component, not from the cascaded css. Net effect: {"type":"card", "style":{"color":"#ff0000"}} wrapping a text with no colour of its own renders white, not red — the documented CLAUDE.md feature "cascade.rs — Héritage color/font-* parent → enfant" has no observable effect. The regression test card_color_cascades_to_text_child_with_no_color_of_its_own (box_builder.rs:3066) passes because it only asserts on the intermediate text_box.css.color field, never on painted output.

Fix

Either thread the cascaded style to the painters — pass _css through PaintCtx (or add a css: &CssStyle parameter to paint_content) and have text-like painters prefer it over self.style — or apply inherit_from to the component's own style field before the painter reads it. Then add a pixel-level test asserting a card's color actually tints a child text, since the existing box-tree assertion cannot catch this class of regression.

Evidence the audit read

fn dispatch(
        &self,
        canvas: &Canvas,
        payload: &(dyn std::any::Any + Send + Sync),
        _css: &rustmotion_core::css::CssStyle,
        layout: &BoxLayout,
        frame: &PaintFrame,
    ) {

Stacked on fix/default-size-font-context, which carries the previous finding of this workstream. GitHub shows only this finding's diff; merge in order.

Part of the September 2026 audit remediation chantier. Refs #220 (RM-07).