Undo/redo reducer

Write historyReducer(state, action) over the shape { past: [], present, future: [] }:

1. { type: 'SET', value } — pushes current present onto past, sets new present, clears future
2. { type: 'UNDO' } — moves present to future, pops the last past entry into present
3. { type: 'REDO' } — inverse of UNDO
4. UNDO with empty past / REDO with empty future return state unchanged
5. Pure function — no mutation, no side effects