feat(char): add rotate_from, scale and baseline jitter, and reflow

[LeadcodeDev]

Refs #429. Part of #438. Two gaps stated below — read them before closing the issue.

The three parts of #363 that were never delivered, all on CharAnimationTiming,
following the pattern scale_from and ink_from already set.

rotate_from tilts each unit and straightens it to zero by the end of that
unit's own animation. rotate_origin picks the pivot: center, or edge for the
leading-edge hinge that reads as a staircase. It is excluded from char_rotate_in
for the same reason scale_from is excluded from char_scale_in — that preset
already owns the axis.

scale_jitter and baseline_jitter give each unit a deterministic size and
baseline offset that decays to zero, so a line is born uneven and settles. They
are independent of jitter, which only perturbs timing, and the three draw from
the same hash under different salts so they do not move in lockstep for one seed.

reflow changes what a unit contributes to its line's width rather than what
it paints: a unit that has not started contributes nothing, and a starting unit's
contribution eases in under reflow_easing. A centred line therefore stays centred
while it is written, instead of reserving its final width from the first frame.

The typewriter complaint does not reproduce

The issue says a centred typewriter grows from a fixed left edge. Probed against
the real pipeline, it does not: text.rs takes align_width from taffy's own
layout.width, which is stable, so centred growth is already symmetric. The
asymmetry exists only in a non-finite-width fallback branch that taffy never
produces. reflow is therefore a new capability here rather than a repair, and
the char_fade_in + reflow pattern is the documented way to get it.

⚠️ Two gaps, named rather than shipped quietly

reflow is accepted and inert on rich_text. The field lives on the shared
CharAnimationTiming, so the schema takes it, but rich_text's own char-animation
renderer precomputes token positions and pill-background bounds independently and
was not touched. That violates this repository's rule that a field the validator
accepts must have an effect. It needs either the implementation or a validator
refusal before #429 closes — I am handling it separately and this PR does not
claim it.

reflow on typewriter is not implemented. AnimationEffect::Typewriter
carries a bare AnimationTiming, read by an exhaustive match in
validate_schema.rs that the workstream could not touch. Putting reflow on the
shared AnimationTiming instead would make it silently inert on every other
preset, which is the same violation. Given that the complaint does not reproduce,
leaving it out looks right — but it is a decision, not an omission.

Tests

Six render tests — scenario, layout, paint, read pixels — each reverted and
watched go red:

rotate_from must visibly tilt the unit mid-animation: plain width=12px, tilted width=12px
scale_jitter must visibly perturb a unit's size mid-animation
reflow must keep the growing centred line's ink centred on the box: box center=350, got ink center=204

rotate_from_also_reaches_gradient_text_through_the_shared_render_path proves the
three new knobs reach gradient_text with no change to that file, because both go
through the shared intrinsic::apply_text_anim_preset.

cargo test -p rustmotion-core --lib 715 passed · -p rustmotion-components --lib 333 · --tests all green.