[LeadcodeDev]

Closes #159.

Measured first, because the issue asked for that

prepare_scene ran on the per-frame path with no cache, although its result
depends only on the scene. Every frame cloned the whole serde_json subtree of
every child and deserialized it into ChildComponent — an untagged enum with 57
variants, so serde re-parses the same object once per failed variant before it
lands.

Timing the call for every frame of a render, release build:

Scenario	before	after
examples/mega-showcase.json — 1236 frames, 9 scenes, 31 children	144 ms	46 ms
50 nodes in one scene, 1200 frames	388 ms	3.2 ms

The 46 ms that remain on mega-showcase are the nine first-time deserializations —
the work that actually has to happen.

End to end this is within noise on both (9.0 s and 8.5 s): the redundancy was
1.6 % and 4.5 % of a render, spread across workers. That is the order of magnitude
the issue asked for before deciding how much to spend, and it is why this is the
contained fix rather than the restructuring the issue sketched. It grows with node
count, which is where it will matter.

Shape

The memo is a OnceLock on the Scene, not a keyed cache. So it has no
identity problem and no invalidation to get wrong: it dies with the scenario.

rustmotion-core cannot name ChildComponent, which lives a crate above it, so
the slot holds Arc<dyn Any + Send + Sync> and prepare_scene is the typed
accessor — and its only writer, which is what makes the downcast total.

Checked before relying on it: the studio rebuilds a whole ResolvedScenario on
every optimistic edit and swaps the Arc, so its scenes are new objects with
fresh slots. include.rs's &mut Scene runs at load, before any render. No path
mutates scene.children in place afterwards.

The warning the issue mentions

deserialize_children emits a stderr line per unreadable child. That fired once
per frame, riding on the same repetition. It is now once per scene, pinned by
a_child_that_cannot_be_read_is_reported_once_rather_than_once_per_frame.

Tests

Four, on Arc::ptr_eq — which is the direct statement of "deserialized once".
Reverting to an uncached Arc::new(deserialize_children(scene)) fails two of them.

Gate

cargo fmt --all --check clean · cargo clippy --workspace --all-targets --features rustmotion/studio -D warnings clean · cargo test --workspace 1802 passed.