Fix five-second clip SFX

[aouxwoux]

What changed

• resolves synthesized WAV paths from the standalone demo executable instead of the process working directory
• keeps the asynchronous filename buffer alive for Windows playback
• replaces the silent five-second X clip with a deterministic SFX mix using the project's original shotgun, explosion, boss, and chaingun sounds

Why

The standalone capture was launched outside the repository root, so relative assets/*.wav paths failed. The MP4 therefore had an audio track but effectively no game sound.

Impact

The public five-second 1080p60 clip now contains clear, synchronized combat SFX, and standalone demo launches can locate their audio assets reliably.

Validation

• lake build lean64 lean64demo
• lean64.exe --smoke-test
• exact video duration: 5.000 seconds / 300 frames / 60 FPS
• measured audio: -14.8 dB mean, 0.0 dB peak, 2.79 seconds above -40 dB