char_* presets: rotate_from, scale/baseline jitter, reflow (undelivered parts of #363)

[LeadcodeDev]

Context

#363 was closed after fixing char_* on rich_text/gradient_text, but three requested capabilities were not delivered and are rejected as unknown fields. rules/char-animation-rich-and-gradient-text.md only notes them as absent.

To do

• rotate_from (+ rotate_origin): each unit starts tilted and straightens while it settles (staircase word reveal).
• scale_jitter / baseline_jitter with seed: deterministic per-unit size and baseline variation (letters born at different sizes, then aligning).
• reflow / reflow_easing on char_* presets and typewriter: a centred line stays centred while it is being written, instead of growing from a fixed left edge.

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.

[LeadcodeDev]

All three items are on CharAnimationTiming on main — rotate_from/rotate_origin, scale_jitter/baseline_jitter with seed, and reflow/reflow_easing (merged in #450).

The two gaps #450 asked to be read before closing are both resolved, each the way the repository rule allows — a field the validator accepts must have an effect, or the validator rejects it or warns:

• reflow on rich_text is named rather than silently inert. check_reflow_reaches_this_component (merged in fix(validate): stop the false positives, name the right edge, warn once #449) emits reflow has no effect on rich_text, because that component's char animation lays out from token positions it precomputes independently.
• reflow on typewriter is rejected outright, verified just now: unknown field reflow, expected one of delay, duration, loop, …, and the component is refused rather than silently dropped. AnimationEffect::Typewriter carries a bare AnimationTiming, and putting reflow there would make it inert on every other preset — the same violation, moved.

Whether typewriter should gain real reflow is a feature question, not this issue's unfinished business: #450 recorded that the original complaint did not reproduce. Open a new issue if a scenario shows it.