fix(components): revive white-space, rich_text measurement and the glow effect #109

Part of #102 — wave 2.

Mini-spec

Goal: revive three features the schema advertises and the engine ignores.

Invariants

• A property the schema accepts either has an observable effect, or is removed. No third state.
• rich_text renders in normal flow without the author having to guess a width.

Out of scope: the geometry validator (owned by the diagnostics workstream this wave), schema type deletion (dead-types workstream).

Acceptance

• M1 — white-space has no render effect. It is read in exactly two places repo-wide: cascade inheritance and the geometry validator. TextIntrinsic::from_parts hardcodes wrap: true with a comment saying CssStyle has no wrap field, and the painter wraps unconditionally at layout_width. So unwrappable_text_overflow fires on a condition the renderer cannot produce.
  Implement nowrap/pre in measurement and painting, so the property means something and the existing validator check becomes truthful. Prove it by rendering: a nowrap line must stay on one line and visibly exceed its box.
• M2 — rich_text is invisible in flow. It has no entry in component_intrinsic, so it lays out 0×0 unless given explicit width/height. Add a RichTextIntrinsic. Note line-breaking currently happens only at span boundaries — a long single span never wraps internally; fix that too or document it precisely.
• M3 — the glow animation effect is a no-op. extract_effects stores it; resolve_props_for_effects never reads it. Renders with and without are byte-identical. Either make it render (note GlowConfig.color is currently never plumbed anywhere, and the keyframe path builds a DropShadow with color: None, which resolves to black), or remove it from the schema and say so. Do not leave it half-alive.

Files: crates/rustmotion-components/src/{box_builder,intrinsic,text,rich_text}.rs, crates/rustmotion-core/src/engine/animator.rs

Verify

rtk cargo test --workspace
rtk cargo build --release
for f in examples/*.json; do ./target/release/rustmotion validate -f "$f"; done

Render-based proof required for each of the three — a passing unit test is not evidence that something is visible on screen.