Use Stopwatch for LockableContext acquire timeouts

[unsafePtr]

DoManualTryLock and DoManualTryPromoteLock use DateTime.UtcNow.Ticks deltas to enforce the caller-supplied TimeSpan timeout. A wall-clock adjustment (NTP step, manual change) lands the comparison on a perturbed value: a backward step keeps retrying past the intended deadline, a forward step gives up early. Both methods are pure local timeout loops — no protocol or persistence concerns — so monotonic time is the right primitive.

Switch to Stopwatch.GetTimestamp() / Stopwatch.GetElapsedTime for the elapsed-time math. Behavior under stable clocks is unchanged; the timeout contract is now enforced against a clock that can't move sideways.

Same NTP-resilience reasoning was applied recently in dotnet/runtime#127303 for EventCounter's polling timer.

Changes

• DoManualTryLock and DoManualTryPromoteLock in LockableContext.cs capture Stopwatch.GetTimestamp() at entry and gate retries on Stopwatch.GetElapsedTime(startTimestamp) > timeout.

Test plan

• TryLockTimeSpanLimitTest, TryLockCancellationTest, TryPromoteLockTimeSpanLimitTest, TryPromoteLockCancellationTest pass on net8.0 and net10.0.
• Tsavorite.core builds clean.
• CI

[Copilot]

Pull request overview

This PR updates Tsavorite's manual lock-acquisition retry loops to measure elapsed timeout using Stopwatch instead of DateTime.UtcNow, so lock and lock-promotion timeouts are based on monotonic time and are no longer affected by wall-clock adjustments.

Changes:

• Replaced wall-clock timeout tracking in DoManualTryLock with Stopwatch.GetTimestamp() / Stopwatch.GetElapsedTime.
• Replaced wall-clock timeout tracking in DoManualTryPromoteLock with the same monotonic timing pattern.

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[kevin-montrose]

This LGTM - please fix conflicts and get a green CI run.