refactor(schema): remove the remaining LayerStyle-era shadow types #111

Part of #102 — wave 2.

Mini-spec

Goal: finish the dead-type removal wave 1 started. Every type left here shadows a live CSS type with different casing or shape, which is what kept stale vocabulary alive in the documentation.

Invariants

• Nothing is deleted without repo-wide grep evidence that it is unreferenced.
• No behaviour change — this is pure removal.

Acceptance

• schema::GridTrack (3 variants: Px/Fr/Auto, externally tagged) is the stale shadow of the live css::style::GridTrack (Fr/Keyword/Length/Minmax, untagged, used by taffy_bridge). Wave 1 could not remove it: it is re-exported through traits/container.rs → traits/mod.rs, outside that wave's file partition. That re-export chain itself appears to have zero consumers — verify, then delete the chain or repoint it.
• schema::style::Spacing and schema::style::GridPlacement became unreferenced once LayerStyle was removed in wave 1. Confirm and delete.
• Re-check for any other orphan left by the LayerStyle removal.

Files: crates/rustmotion-core/src/schema/{style,video}.rs, crates/rustmotion-core/src/traits/{container,mod}.rs

Verify

rtk cargo test --workspace
rtk cargo build --release
./target/release/rustmotion schema | jq '.definitions | keys | length'

Paste the grep output proving each deleted item was unreferenced.