fix(components): wire Caption and Positioned into timed/animatable dispatch

[LeadcodeDev]

Goal: remove the palette asymmetry — as_timed returns None for Caption and Positioned (crates/rustmotion-components/src/lib.rs:289), as_animatable returns None for Positioned.

Invariants: every component that carries timing/animation fields is reachable through the dispatch; defaults keep current behavior.

Acceptance: a Positioned with a fade-in animation animates; a Caption with start_at respects its window; tests cover both.

Files: crates/rustmotion-components/src/lib.rs, caption.rs, positioned.rs.

Verify: cargo test --workspace