PR-18 feat(worker): define worker protocol

[HsienW]

Objective

Define explicit, versioned messages between main thread and worker before writing worker runtime logic.

Main → Worker

{ type: 'COMMAND_BATCH', seq: 42, commands: [...] }

{ type: 'RUNTIME_CONTROL', action: 'PAUSE' }

Worker → Main

{ type: 'SNAPSHOT', tick: 123, state: {...}, metrics: {...} }

{ type: 'GAME_EVENT', event: {...} }

Ownership rule

• Worker owns: tick, snake positions, collision, food, score, seeded RNG, replay log.
• Main thread owns: keyboard/browser input collection, buttons, accessibility UI, current renderer, main-thread performance panel.
• No shared mutable state in the first Worker version.

Exit criteria

• Worker protocol is versioned