2048-cross-platform-parity

# 2048 cross-platform parity Read `AGENTS.md`, `docs/architecture.md`, and the relevant platform skills. Write down the observable behavior before editing, then locate its engine/ViewModel, presentation, persistence, and tests in every affected client. Preserve the shared rules contract while using native presentation patterns: SVG and browser semantics on web, SF Symbols and SwiftUI accessibility on iOS, Material icons and Compose semantics on Android. Pixel identity is not required; equivalent capability, hierarchy, and feedback are. Server-driven surfaces are a parity surface of their own. A node type must exist on both native clients before a payload uses it, the two bundled payloads stay byte-identical, and every call site keeps its native fallback. Surfaces describe content only — rules, styling, and behaviour stay in code. Add parallel deterministic cases for rule changes and platform-appropriate UI coverage for flow changes. Run `make check` and each affected `make test-*` target. Report any unavailable toolchain or intentional divergence explicitly instead of treating one passing client as proof for all three.