feat(clock): add TimerSkippingRealExecutionClock

[janrnc]

• Adds TimerSkippingRealExecutionClock, a manual clock that skips idle time like the existing instant-execution clock, but lets triggered jobs run for real in their own goroutines.
• While any job is executing, the virtual clock flows 1:1 with the wall clock so jobs observe real time passing and any activation that comes due fires naturally; while idle, it freezes and jumps straight to the next activation or the requested target.
• AdvanceTo/AdvanceBy return once the clock reaches the requested instant (paying the real execution time of any job windows crossed along the way); WaitForIdle blocks until all triggered jobs have completed and the clock has frozen again.

[codecov]

Codecov Report

❌ Patch coverage is 95.98214% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.34%. Comparing base (c4e762c) to head (bef33aa).

Files with missing lines	Patch %	Lines
timer_skipping_real_execution_clock.go	95.98%	6 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
+ Coverage   95.18%   95.34%   +0.15%     
==========================================
  Files          18       19       +1     
  Lines         893     1117     +224     
==========================================
+ Hits          850     1065     +215     
- Misses         30       36       +6     
- Partials       13       16       +3     

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