Cardano Updates

Each shard runs its own SequentialSimulation on a dedicated OS thread
(std::thread::scope), with CMB ceiling enforcement via AtomicTimestamp
and cross-shard messages via std::sync::mpsc channels.

With 6 shards on realistic 3000-node topology:
- Sequential+CMB: 2m28s wall (16 cores utilized)
- Actor+CMB:      3m45s wall (10 cores utilized)
- 1.5x speedup over actor sharding, 4x over actor single-shard

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Replace the actor-based simulation with a synchronous event loop for
single-shard runs. The sequential engine eliminates tokio coordination
overhead (channels, oneshot allocs, task scheduling) by processing
events directly from a global priority queue.

Events at the same timestamp are batched and processed in parallel
across nodes using rayon, following a Bulk Synchronous Parallel model:
pop batch → resolve deliveries → parallel node compute → apply results.

Enabled by default for single-shard (sequential-engine: true in config).
Falls back to sequential processing for small batches (<32 events).
The actor engine remains available via sequential-engine: false.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Signed-off-by: cryptodj413 <[email protected]>