test: add engine state immutability coverage

[rlippmann]

Summary

• add test-only coverage for engine state external immutability
• verify mutating engine.state snapshots does not affect internal engine state
• verify constructor input object is not aliased after createEngine({ state })
• verify mutating decision.state does not affect internal engine state

Scope

• tests only
• no engine/runtime changes

Verification

• npm test -- tests/engine_state_immutability.test.ts
• npm test