Add high-value property tests for HLC and vector clocks

[Copilot]

Adds FsCheck-based property tests for HlcTimestamp and VectorClock to verify critical distributed systems invariants.

HlcTimestampProperties.fs:

• ToPackedInt64 truncates counter and node id — verifies packed 64-bit encoding masks Counter to 12 bits and NodeId to 4 bits (and preserves wall time in the tested domain).
• ToString and Parse round-trip — Parse(ts.ToString()) = ts.
• ToString and TryParse round-trip — TryParse(ts.ToString()) succeeds and yields original value.
• TryParse is non-throwing — TryParse(string?, out VectorClockMessageHeader) never throws for arbitrary input strings (and if it returns true, parsed.ToString() matches the input).
• Operators are consistent with CompareTo — <, >, <=, >= results match CompareTo(HlcTimestamp).
• WriteTo encoding preserves lexicographic ordering — big-endian 10-byte encoding compares lexicographically the same way as CompareTo(HlcTimestamp).

VectorClockProperties.fs:

• Merge returns the upper bound when one input dominates — if a <= b then a.Merge(b) = b, if a >= b then a.Merge(b) = a, concurrent case ignored.
• Merge is monotone with respect to inputs — merged result is never Before either input.
• Increment is stable under merge with original — clock.Merge(clock.Increment(nodeId)) = incremented and symmetric.

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