Performance + maintainability improvements for HLC [timestamps, headers, coordinator, GUID factory]

[Copilot]

Optimizations for HLC hot paths and file organization improvements. All changes preserve HLC algorithm semantics while reducing allocations and improving code clarity.

HlcTimestamp

• WriteTo/ReadFrom: Use BinaryPrimitives for counter/node serialization
• Add Parse(ReadOnlySpan<char>) for allocation-free parsing from header segments
• Refactor ToPackedInt64() with explicit ulong intermediates to reduce casts

HlcMessageHeader

• ToString(): Use string.Create + Guid.TryFormat("N") instead of concatenation
• Parse(): Span-based parsing eliminates Split() allocations

HlcStatistics

• Use long.Abs(delta) to avoid Math.Abs(long.MinValue) overflow edge case

HlcGuidFactory

• Increase RNG buffer 64→256 bytes to reduce refill frequency under lock (4× improvement)
• Witness(HlcTimestamp): Eliminate temp struct allocations by inlining max selection logic
• Witness(long): Replace nested Math.Max with sequential comparisons
• CreateGuidFromHlc(): Use BinaryPrimitives for counter/node packing while preserving version/variant bits

File organization
Split HlcCoordinator.cs supporting types into dedicated files:

• HlcStatistics.cs
• HlcMessageHeader.cs
• HlcClusterRegistry.cs

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