Code review: HLC sign-extension fix, VectorClock optimizations, deterministic demo improvements

[Copilot]

Reviewed 5 commits implementing performance optimizations and correctness fixes for HLC/VectorClock infrastructure and deterministic simulation demo.

Key Changes Reviewed

HLC packed encoding (bf427b7)

• Fixed sign-extension bug in FromPackedInt64 when bit 47 is set
• Now uses unchecked((ulong)packed) with explicit masking to prevent sign extension
• Added test coverage for high-bit wall times

VectorClock canonicalization (9932afd)

• Replaced sort-and-dedupe with CollectionsMarshal.GetValueRefOrAddDefault pattern
• Eliminates redundant dictionary lookups and sorting overhead in CreateCanonical

VectorClockBuilder pooling (0106037)

• Switched from heap allocation to ArrayPool<byte>.Shared.Rent for merge buffers
• Uses stackalloc for buffers ≤ 1024 bytes, pool for larger
• Reduces GC pressure on hot merge paths

VectorClockCoordinator integration (e0503f1)

• Added VectorClockBuilder class to enable efficient in-place merge/increment
• Updated coordinator to reuse builder instance across operations
• Added IsEmpty property and internal snapshotting helpers

Deterministic demo (60fadc4)

• Switched from ConcurrentDictionary to Dictionary/HashSet (single-threaded by design)
• Reworked network to use PriorityQueue with deterministic tie-breaking
• Models reordering as bounded jitter clamped to delivery time ≥ send time

Review Summary

✅ All changes maintain wire format compatibility
✅ Tests pass (67/67)
✅ Build succeeds
✅ Performance improvements align with "systems library" standards goal

Minor observation: CollectionsMarshal usage in canonicalization is performant but advanced—consider brief comment explaining the optimization for maintainability.

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