Enforce HLC drift bounds on backward clock + harden vector clock parsing

[Copilot]

Tightens HLC drift enforcement to detect backward clock jumps immediately, and hardens VectorClock parsing against malformed input with extra separators.

Changes

• HLC drift checking: NewGuidWithHlc() and Witness() now validate drift bounds after every state update, catching large backward-clock jumps immediately when ThrowOnExcessiveDrift=true
• VectorClock parsing: Entry parsing now splits on first : only (Split(':', 2, ...)) to avoid issues with malformed input containing extra separators
• VectorClock whitespace: Parse methods treat whitespace-only strings as empty input
• Lock consistency: VectorClockCoordinator uses Lock type consistent with other coordinators
• Drift test coverage: Added regression tests for backward-clock drift enforcement

Example

// Backward clock now detected immediately
var tp = new SimulatedTimeProvider();
var factory = new HlcGuidFactory(tp, 
    options: new HlcOptions { MaxDriftMs = 0, ThrowOnExcessiveDrift = true });

factory.NewGuidWithHlc();  // logical time = 1000
tp.SetUtcNow(tp.GetUtcNow() - TimeSpan.FromMilliseconds(1));  // clock jump backward

factory.NewGuidWithHlc();  // ❌ throws HlcDriftException immediately

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