Improving cancellations in the parallel builder.

**### Summary**
We need a robust way to handle cancellations (or updates) in the parallel builder, where drain_new_orders() returns None. Because sub-components (`ConflictFinder`, `ResultsAggregator`, etc.) store partial merges and simulation states, a single order removal is insufficient. Instead, we must fully reset them all, wiping stale references. However, the builder also spawns threads (worker pool, aggregator, block-building), which complicates concurrency.

### Goals
1. **Full Reset**: When cancellations occur, we must clear:
* ConflictFinder
* ConflictTaskGenerator's `existing_groups` / `task_queue`
* ConflictResolvingPool's `task_queue`
* SimulationCache
* ResultsAggregator’s best_results
Then re-ingest only orders and let the system proceed as if new (e.g. find conflicts, resolve them, build blocks).

2. **Thread Safety**: Ensure no data races or partial updates if aggregator/block-building threads hold references.
3. **Maintain High Performance**: Avoid large overhead from locking or repeated resets.

### Core Considerations
1. **Reset Mechanism**: All submodules (conflict finder, aggregator, simulation cache, etc) need a consistent way to clear and re-ingest orders—no partial or stale references.
2. **Concurrency Approach**:
*. **Single-Thread Manager**: Central struct (`ParallelBuilder`) mutates data, with worker threads taking only short-lived snapshots. Simple to reset, but limits fully parallel reads.
*. **Multi-Thread Shared State**: Modules live behind locks (Arc>) or receive message-based commands. More flexible for continuous concurrency, but demands locking or event loops.
3. **Data Usage Pattern**: Decide if aggregator/block-building hold references 24/7 (requiring concurrency control) or just use ephemeral tasks (easier single-thread reset).
4. **Performance vs. Simplicity**: Locking or message-passing ensures safety for truly parallel modules, but increases complexity and overhead. A single-thread manager is simpler but less concurrent.

Please weigh in on the core considerations.