Recording (Windows): raise the timer resolution and register capture threads with MMCSS #921

Part of #920 (rank 1). Effort S, risk low.

Problem

• No Windows helper raises the timer resolution, registers with MMCSS or sets a thread priority. Every wait lands on the default 15.625 ms tick.
• Affected waits: the video writer's sleep_until (main.cpp), the WASAPI poll (wasapi_loopback_capture.cpp, 5 ms asked), the audio mixer (audio_sample_utils.cpp), cursor-sampler.cpp.
• Measured on a real take (recording-1790792371876):
  • webcam frame intervals are multiples of 15.625 ms (±0.4 ms), 13.6 % duplicates, 25.9 unique fps for 30;
  • cursor sampled at 22.7 Hz for 30 Hz asked (47 ms median, 3 ticks);
  • late WASAPI packets are the upstream cause of the Audio: low-frequency crackle or saturation heard in the editor, possibly on loud input #911 audio holes.

Proposal

• timeBeginPeriod(1) for the life of the recording (and timeEndPeriod on exit) in wgc-capture and cursor-sampler.
• Opt the process out of power throttling (SetProcessInformation(ProcessPowerThrottling)), since Windows 11 can ignore the timer request of a throttled process. High-resolution waitable timers (CREATE_WAITABLE_TIMER_HIGH_RESOLUTION) are the alternative.
• Register the capture, mixer and writer threads with MMCSS (AvSetMmThreadCharacteristicsW, "Capture" / "Pro Audio").

Verify

• Same 60 s take before and after: webcam interval histogram (>95 % at 33 ms ±2, duplicates <2 %), cursor at 30 Hz ±1, [pacing] frames/elapsed ≥ 59.5 at 60 fps.
• Repeat with the app window minimized.