fix: make FFmpeg runtime build portable

[abrichr]

Fixes the managed-runtime publication failures without weakening the release boundary.

• Materializes the raw RGB smoke fixture as bytes before writing it.
• Exercises Capture 1.0.3’s real staged-PNG ffconcat input and image2pipe decode output.
• Converts GitHub Windows source, output, and temporary drive paths to MSYS paths.
• Builds the native .exe Make targets on Windows.
• Uses the exact single-backslash FFmpeg select expression.
• Keeps Windows on the verified portable software MPEG-4 encoder; the broken Media Foundation encoder is not advertised or built.
• Adds focused regression assertions for every portability contract.

Local verification: 4 focused tests, Ruff, shell syntax, diff checks, and real encode/decode smoke pass.

Failure evidence runs:

• https://github.com/OpenAdaptAI/openadapt-desktop/actions/runs/30053682152
• https://github.com/OpenAdaptAI/openadapt-desktop/actions/runs/30054171654
• https://github.com/OpenAdaptAI/openadapt-desktop/actions/runs/30054549110

Exact four-platform non-publishing proof:

• https://github.com/OpenAdaptAI/openadapt-desktop/actions/runs/30054904040