Code review: VectorClock optimizations and deterministic simulation refinements

[Copilot]

Reviewed PR changes that optimize VectorClock coordinator allocation patterns and refine the deterministic distributed simulation demo.

Review Findings

VectorClock changes:

• New internal VectorClockBuilder reduces allocation churn on hot paths (BeforeSend/BeforeReceive/NewLocalEvent)
• IsEmpty property provides explicit empty-state check vs. default equality comparison
• Binary format documentation clarified (big-endian, canonical sorted output)

Demo refinements:

• Replaced ConcurrentDictionary → Dictionary and HashSet (appropriate for single-threaded deterministic simulation)
• Network scheduling upgraded to PriorityQueue with sequence-based tie-breaking for deterministic ordering
• Reordering modeled as bounded delivery-time jitter (clamped ≥ now) rather than position swaps

Validation:

• All 67 property tests pass
• Distributed showcase demo runs with expected behavior
• Wire formats and semantics preserved

The changes achieve stated goals of improving efficiency and explicitness while maintaining backward compatibility.

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