Animations touching only border_radius, clip_path_progress or font_size are never applied #426

Context

Found while verifying #373 and #384 (both closed), and reported independently by a user for font_size.

Cause

props_has_paint_overrides (crates/rustmotion-components/src/box_builder.rs, around line 1039) is an allow-list (translate, scale, rotation, opacity, blur, glow, perspective, width, height). apply_animated_props is only called when it returns true, so an animation that touches only a property missing from that list is dropped. Adding any translate_x track to the same node "unlocks" it.

Repros

• Animating border_radius validates and has no effect #373: {"type":"div","style":{"width":200,"height":200,"background":"#000000","animation":[{"name":"keyframes","keyframes":[{"property":"border_radius","keyframes":[{"time":0,"value":100},{"time":1,"value":100}]}]}]}} → sharp corners at frame 15. On a shape rect, corners stay sharp even with another track present.
• Animate clip-path: interpolate polygon, inset, circle and ellipse in keyframes #384: the example of rules/clip-path-morph.md (kind: morph + a clip_path_progress track) renders identical frames at t=0, 1 and 2.
• font_size keyframes on a text (40 → 200) are accepted and the text keeps its initial size.

To do

• Make every animated property reach the render, without a hand-maintained allow-list.
• Audit every animatable property for the same defect (letter_spacing, stroke_width, gap, padding, …) with one render test per property animated alone.
• border_radius animation on shape.
• font_size animation applied to text layout and rendering.

Definition of done

• A test that fails on current main is written first. For anything visual it is a render test (scenario → frame → pixel assertions); tests that build AnimatedProperties by hand are not enough — that is exactly how the regressions listed here slipped through.
• Each item is verified end to end: scenario JSON, rustmotion validate, render, image inspected.
• cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, cargo test --workspace pass.
• Vocabulary changes update crates/rustmotion/skills/SKILL.md, the relevant rules/ file and the exported schema. A field the validator accepts must have an effect, otherwise the validator rejects it or warns.
• No comments in Rust code (repository rule). Commits follow <type>: <verb> <message>. No AI attribution in commits, PRs or issue comments.