refactor(geometry): reuse the canonical component_kind helper

[LeadcodeDev]

Severity Low, category quality. Location: crates/rustmotion/src/cli/commands/geometry.rs:1158

Impact

rustmotion_components::box_builder::component_kind (box_builder.rs:2090-2154) is public and already imported elsewhere in the same crate (crates/rustmotion/src/engine/render/scene.rs:744). geometry.rs re-implements it as a private 60-arm copy. Both must be edited for every new component, and both already carry the same two label bugs: QrCode(_) => "qrcode" while the serde tag is qr_code, and Container(_) => "container" while the serde tag is div. Those strings are user-facing — they are the component: field of every GeometryViolation and drive the hint branches at geometry.rs:686 and :1591 — so a violation on a qr_code reports a type name that does not exist in the schema. Exhaustiveness protects against a missing arm but not against the two copies drifting on a rename, which is exactly how the labels got out of sync with serde in the first place.

Fix

Delete the geometry.rs copy and use rustmotion::components::box_builder::component_kind;. In the surviving copy, return the actual serde tags ("qr_code", "div") — or better, derive the label from the schema so it cannot drift from #[serde(tag = "type", rename_all = "snake_case")] at all.

Evidence the audit read

fn component_kind(c: &Component) -> &'static str {
    match c {
        Component::Text(_) => "text",
        Component::Shape(_) => "shape",
        ...
        Component::QrCode(_) => "qrcode",
        ...
    }
}

Stacked on fix/geometry-autoscroll-box, 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-40).