feat(schema): export the component schema, load fonts before validating, drop LayerStyle

[LeadcodeDev]

Part of #102 — wave 1.

Mini-spec

Goal: make the schema surface honest — export a schema that actually describes components, measure text with the fonts the render will use, and delete the dead style model that keeps stale vocabulary alive.

Invariants

• rustmotion schema output fully describes every Component variant.
• Validation and render resolve the same typeface for the same text.
• No public type remains that contradicts the live CSS types on casing or shape.

Out of scope: validate_attrs hardening and SceneEntry diagnostics (wave 2, L4b); the colour parser itself (L2).

Acceptance

• H2 — cmd_schema emits schemars::schema_for!(Component) (already built internally at validate_attrs.rs:40) so Scene.children is no longer typed items: true. The output must contain a oneOf over component variants.
• H5 (fonts) — load_custom_fonts runs inside validation::run_checks, before any measurement. Today it is only called on the render path (render.rs:47), so validation measures through the Helvetica/Arial fallback while the render uses the declared face.
• H8 (code) — LayerStyle (schema/style.rs:317+) and its orphaned siblings that shadow live CSS types (schema::BlendMode, schema::Overflow, schema::GridTrack, CardDisplay, schema::Size) are removed, along with their pub use re-exports. Verify each is genuinely unreferenced before deleting.

Files: crates/rustmotion-core/src/schema/**, crates/rustmotion-cli/src/commands/schema.rs, crates/rustmotion-cli/src/commands/validation.rs

Note: crates/rustmotion-core/src/css/style.rs is a different file owned by L3 — do not touch it.

Verify

rtk cargo test --workspace
rtk cargo build --release
./target/release/rustmotion schema | jq '.definitions | keys | length'
for f in examples/*.json; do ./target/release/rustmotion validate -f "$f"; done

[LeadcodeDev]

Merged into main via #108 (squash 38596ca).