Add subtle chat activity motion primitive

[Afnanksalal]

What changed

• Add chat_activity, an assistant loading bubble with quiet breathing dots and a shimmer rail.
• Keep the effect token-driven and scoped by id_source for stable composition.
• Document the primitive in the README component table.

Verification

• cargo fmt --all
• cargo clippy --all-targets -- -D warnings
• RUSTDOCFLAGS='-D warnings' cargo doc --no-deps