fix(geometry): resolve vw/vh against the scenario viewport

[LeadcodeDev]

Severity Medium, category coherence. Location: crates/rustmotion/src/cli/commands/geometry.rs:180

Impact

ConversionContext::default() carries LengthContext { viewport_width: 1920.0, viewport_height: 1080.0, .. } (rustmotion-core/src/css/units.rs:66-74), and taffy_bridge::size_to_dim/lp_to_lp/lp_to_lp_auto resolve every vw/vh/rem/em against it (taffy_bridge.rs:335-338). The renderer does NOT: render_with_new_pipeline_iter (engine/render/scene.rs:554-558) and render_scene_hits (scene.rs:792) both pass viewport_conversion_context(vw, vh), whose own doc comment calls the 1920×1080 default a 78% error on a 1080×1920 vertical video. So on any non-1920×1080 scenario the mandatory rustmotion validate gate checks a layout the renderer never produces: width: "50vw" is measured as 960px by the validator and painted as 540px, 50vh as 540px vs 960px. The file header promises the opposite ("so the geometry it checks matches what the renderer will actually paint"). The same file builds a correct LengthContext with the real viewport at line 477 for its transform-origin math, so the omission is confined to the layout pass. Both the resting walk (:180) and the --strict-anim walk (:1347) are affected.

Fix

Make viewport_conversion_context public (or move it next to run_layout in rustmotion-core) and call it from both geometry.rs sites: run_layout(&built.root, viewport_f, &viewport_conversion_context(viewport_f.0, viewport_f.1)). Better: stop exposing ConversionContext at the run_layout signature at all — derive it from the viewport argument inside run_layout, which makes the wrong context unrepresentable.

Evidence the audit read

let root_css = render::root_style(scene.layout.as_ref(), view.view_type.clone());
let built = build_scene_from_refs(children.iter(), viewport_f, root_css, None);
let layouts = run_layout(&built.root, viewport_f, &ConversionContext::default());

Based directly on the chantier branch.

Part of the September 2026 audit remediation chantier. Refs #220 (RM-05).