char_* presets: rotate_from, scale/baseline jitter, reflow (undelivered parts of #363) #429

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.