fix(layout): hand IntrinsicMeasure both sizes in the content box

[LeadcodeDev]

Severity Low, category correctness. Location: crates/rustmotion-core/src/engine/layout_pass.rs:97

Impact

taffy 0.10.1 (compute/leaf.rs) subtracts content_box_inset (padding+border) from available_space before calling the measure fn, but passes known_dimensions — the outer border-box size — untouched. The two arguments are therefore in different coordinate spaces, and neither IntrinsicMeasure's trait doc (box_tree.rs:114-122) nor this closure says so. TextIntrinsic::measure (rustmotion-components/src/intrinsic.rs:224) prefers known.0 as its wrap width, while Text::paint wraps at layout.content_box() width (legacy_dispatch.rs:145-155). A text with horizontal padding is therefore measured at a wider line width than it is painted at: fewer wrapped lines, a reserved box one line too short, and the text spills out of its own box. The geometry validator re-measures through the same intrinsic, so it signs the overflow off. intrinsic.rs:258-265 explicitly asserts the opposite ("taffy hands a leaf its own known/available height already padding/border-subtracted") — true for available, false for known.

Fix

Use the _style argument already handed to the closure to subtract resolved padding+border from known before calling intr.measure, so both arguments are content-box. Then state that invariant on IntrinsicMeasure::measure in box_tree.rs, and add a regression test for a padded text (measured height vs painted line count).

Evidence the audit read

|known, available, _node, ctx_data, _style| {
    let Some(ctx) = ctx_data else { return tf::Size::ZERO; };
    let Some(intr) = ctx.intrinsic.as_ref() else { return tf::Size::ZERO; };
    let (w, h) = intr.measure(
        (known.width, known.height),
        (available.width.into(), available.height.into()),
    );

Stacked on fix/em-font-size, which carries the previous finding of this workstream. GitHub shows only this finding's diff; merge in order.

Part of the September 2026 audit remediation chantier. Refs #220 (RM-27).