fix(html): reject unknown scene attributes instead of dropping them

[LeadcodeDev]

Severity Medium, category coherence. Location: crates/rustmotion-html/src/scene.rs:20

Impact

Executed: <scene duration="2" freeze_at="1" world-position="0,0" animated-background="halo"> transpiles to a scene carrying only duration and layout — three documented scene features (CLAUDE.md documents all three) are discarded with no warning and validate exits 0. Same on the root: <rustmotion codec="prores" durationn="5"> drops both. Same on containers/text: <div gap="32" width="400" id="x"> keeps nothing but style/anim (element.rs:101-112 and :92-100 never iterate attrs). This is the exact failure mode check_component_attrs exists to prevent, but that check runs on the already-transpiled ResolvedScenario (validate_attrs.rs:79 check_component_attrs(scenario: &ResolvedScenario)) and only over scene.children — an attribute the transpiler never emitted is invisible to it, so the workspace's only unknown-attribute net has a blind spot precisely over the HTML front-end's own attribute surface. rm-* custom elements are the asymmetric exception: they forward everything and do get caught.

Fix

After reading the known keys in scene_to_value and html_to_scenario_value, diff against the consumed set and return a named error (with did-you-mean, mirroring validate_attrs) for the remainder — class/id/data-* can be allowlisted as deliberately inert. Do the same for TagKind::Container/TagKind::Text. Separately, extend <scene>'s allowlist to cover freeze_at, world-position and animated-background, which are otherwise unreachable from HTML.

Evidence the audit read

let attrs = element_attrs(handle);
    let get = |k: &str| attrs.iter().find(|(n, _)| n == k).map(|(_, v)| v.clone());

    let duration = get("duration").ok_or(HtmlError::MissingDuration)?;
// only duration/align/justify/direction/gap/padding/background/effects/
// transition{,-duration,-easing} are ever read; nothing iterates `attrs`
// to report the leftovers.

Based directly on the chantier branch.

Part of the September 2026 audit remediation chantier. Refs #220 (RM-03).