Add ThrottleLock

[Copilot]

• Switch to monotonic time source (Stopwatch.GetTimestamp() / GetElapsedTime())
• Fix double timestamp capture (resolved by monotonic switch)
• Consolidate timing tests into 2 focused tests
• Use is { } last pattern for nullable long? check (superseded — field is now non-nullable)
• Redesign WaitAsync_Test: 50ms interval, 3 sequential waits, assert >= interval * 3; initialize _lastTimestamp at construction so every call is throttled
• Fix CSharpier formatting issue that was causing CI build failure

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.93%. Comparing base (45628b8) to head (29359f5).
⚠️ Report is 2 commits behind head on prime.

Additional details and impacted files

@@            Coverage Diff             @@
##            prime      #70      +/-   ##
==========================================
+ Coverage   93.85%   93.93%   +0.07%     
==========================================
  Files         128      130       +2     
  Lines        2524     2557      +33     
  Branches      200      202       +2     
==========================================
+ Hits         2369     2402      +33     
  Misses        112      112              
  Partials       43       43              

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[Copilot]

Pull request overview

Ports ThrottleLock from YoutubeDownloader into PowerKit as an internal utility to enforce a minimum interval between consecutive async acquisitions (rate-limiting).

Changes:

• Added ThrottleLock (SemaphoreSlim-backed) with WaitAsync(CancellationToken) to throttle acquisitions to a fixed cadence.
• Added unit tests covering first call behavior, subsequent throttling, post-interval behavior, cancellation, and disposal.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
PowerKit/ThrottleLock.cs	Introduces the ThrottleLock utility and its async throttling logic behind TFM guards.
PowerKit.Tests/ThrottleLockTests.cs	Adds timing-based unit tests validating ThrottleLock.WaitAsync behavior.

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[Tyrrrz]

@copilot please fix formatting