Text: letter_spacing on rich_text/gradient_text, animated tracking layout, static opacity vs animated opacity, caret after typed text

To do

letter_spacing keyframes have no effect on rich_text and gradient_text (Text animation: animatable letter-spacing, and a letter-by-letter morph between two labels #380). rich_text even ignores a static style.letter-spacing.
Animated letter_spacing on text overflows and drifts (Text animation: animatable letter-spacing, and a letter-by-letter morph between two labels #380): layout uses the static value, so tracked-out text overflows its box and grows to the right instead of staying centred.
Static opacity: 0 cancels an opacity animation (Two animations on the same property compose multiplicatively, but the docs promise last-wins #322, closed by a docs-only PR): {"type":"div","style":{"width":100,"height":100,"background":"#FF0000","opacity":0,"animation":[{"name":"fade_in","duration":0.2}]}} stays invisible for the whole scene, with no warning (css.opacity = base * animated in css/animation.rs). The animation should override the static value, or validate should warn.
Caret after typed characters: document (and test) how to place an element right after the characters typed so far during a typewriter — node("id","width") gives the full box width, not the typed width.
Low priority (svg: <text> elements are never drawn, with no warning #374, dropped): fonts declared in fonts are not used by <text> inside an inline svg.

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.