fix(geometry): measure text overflow against the content box

[LeadcodeDev]

Severity Low, category correctness. Location: crates/rustmotion/src/cli/commands/geometry.rs:785

Impact

bbox here is raw_bbox = bbox_of(layout) (geometry.rs:272, 376-383), i.e. layout.width — the BORDER box. But LegacyPaintDispatcher::dispatch (crates/rustmotion-components/src/legacy_dispatch.rs) hands every non-codeblock painter a synthetic BoxLayout { width: cw, height: ch, border/padding: zero } taken from layout.content_box() and translates the canvas to the content-box origin. So Text::paint wraps and draws inside cw, not layout.width. A white-space: nowrap text with style.padding: "0 24px" in a 300px box has cw = 252; a natural line width of 280px paints 28px past the box's right edge, yet 280 > 300 is false and nothing is reported. The sibling check check_content_overflows_box (geometry.rs:848) does this correctly via layout.content_box() — the two checks disagree about which box text lives in, and the flagship unwrappable_text_overflow is the one that is wrong. The gap equals padding.left + padding.right + border.left + border.right.

Fix

In walk/walk_anim, pass layout.content_box()'s (x, y, w, h) to check_unwrappable_text instead of raw_bbox, matching check_content_overflows_box. Keep raw_bbox for check_viewport (which genuinely tests the border box against the frame).

Evidence the audit read

let (natural_w, _) = intrinsic.measure(
    (None, None),
    (AvailableSpace::Definite(bbox.w), AvailableSpace::MaxContent),
);
if natural_w > bbox.w + 0.5 {

Stacked on fix/validate-relative-paths, 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-31).