fix(svg): reject draw: true when nothing drives draw_progress

[LeadcodeDev]

Closes #318.

The premise, verified

Two stills of the same mark, one with draw: true and one without, come back
byte-identical (cmp on the PNGs). The flag changes nothing:

let draw_active = self.draw || (props.draw_progress >= 0.0 && props.draw_progress < 1.0) || ...;
if draw_active {
    let progress = if props.draw_progress >= 0.0 { props.draw_progress } else { 1.0 };
    ...
    if progress >= 1.0 { self.paint_resvg(...); }   // the same call paint_static makes

With nothing driving it, draw_progress sits at its resting sentinel, progress
resolves to 1.0, and the >= 1.0 arm delegates straight to resvg.

Two documents said otherwise

• The field's doc comment: "Force draw-on mode even when draw_progress is 1.0
  (static draw trace view, no animation needed)." There is no trace at progress
  1.0, only the finished render.
• rules/draw-progress-stroke.md listed draw: true as a way to reveal a stroke
  progressively.

Both corrected. The doc comment is schemars-visible, so it is interface text and
stays — it now says what the flag actually does and that it needs a driver.

Why reject rather than reinterpret

The issue offered two readings. Treating an undriven draw as progress 0 makes
the mark vanish — a different surprise, not an answer. Rejecting it names the
problem where the author can act on it, which is the discipline this audit keeps
arriving at.

views[0].scenes[0].children[0]: draw: true but nothing animates draw_progress —
the mark renders finished, pixel-identical to draw: false. Add a 'draw_in' or
'stroke_reveal' preset, or keyframes on 'draw_progress'.

A driver is a draw_in/stroke_reveal preset, keyframes on draw_progress or
draw_start, or a wiggle on either.

Worth knowing

draw: true is not needed even with a driver: the painter switches as soon as
draw_progress is inside [0, 1). The flag only widens that window, so it is
never the thing that makes a stroke animate. The rule file now says so.

Tests

draw_with_nothing_driving_progress_is_pixel_identical_to_no_draw pins the
symptom itself, next to six validator tests covering each driver and each
non-driver.

Gate

cargo fmt --all --check clean · cargo clippy --workspace --all-targets --features rustmotion/studio -D warnings clean · cargo test --workspace 1778 passed.