Fix Zoom audio pacing and responsive meters

[iamfatness]

What changed

• Prime Zoom meeting-mix and participant ISO audio onto the 20 ms program clock with a permanent one-tick reserve.
• Prevent 10 ms partial blocks from reaching Program, then re-prime cleanly after a genuine source gap.
• Preserve the Zoom pacing flag when PCM frames are coalesced.
• Make vertical audio-meter segments fill the available channel-strip height and remain aligned with the dBFS scale.
• Add native and WinUI regression coverage for pacing, re-priming, viewport growth, and compact-window behavior.

Why

Zoom supplies raw audio in 10 ms packets while the CoreVideo program mixer runs on a 20 ms tick. Sending whatever was available allowed partial blocks to reach Program, producing audible artifacts and weakening audio/video sync. Separately, the mixer used fixed-height meter segments, so taller windows created unused space while the dB scale continued to grow.

Operator impact

Zoom ISO and meeting-mix audio now enter Program in complete 20 ms blocks with a stable timing cushion. The mixer meters now scale with compact and maximized layouts instead of remaining pinned to a fixed height.

Validation

• ctest -C Release --test-dir native/build-dev --output-on-failure — passed
• dotnet test native-shell/CoreVideoPro.WinUI.Tests/CoreVideoPro.WinUI.Tests.csproj -c Debug --no-restore — 710 passed
• npm run pack:native — passed; packaged native-core hash matched the validated build
• Local package smoke test — UI and native core responsive
• Visual resize smoke test — meter segments aligned with the dBFS scale in compact and maximized windows

A fresh real-meeting soak remains appropriate before promoting the alpha.