feat: add WithBatchDivergence and replace magic block height in manager_test

[sashass1315]

• Introduce WithBatchDivergence(batch uint64) in the mock L2 state provider to configure divergence by batch number instead of raw block height.
• Derive blockDivergenceHeight lazily during NewForSimpleMachine when the first state with GlobalState.Batch == batch is encountered; remains fully backward-compatible with WithBlockDivergenceHeight.
• Update constructor validation to require WithMachineDivergenceStep when either block- or batch-based divergence is enabled.
• Refactor TestComplexAssertionForkScenario to use WithBatchDivergence(4), removing the non-intuitive magic number (36) and the related TODO.