fix(engine): resolve root-absolute media from project

[miguel-heygen]

Summary

• resolve browser root-absolute media URLs from the project root
• preserve explicit absolute filesystem paths when they exist
• apply identical path semantics to audio mixing and video frame extraction
• add regression coverage for root-absolute audio and video sources

Root cause

Node classified paths such as /.media/tone.wav as filesystem-absolute, so the audio and video pipelines skipped the project-relative resolver. Browser-served images already interpreted the same path from the project root, creating inconsistent media behavior.

Verification

• 13 audio mixer tests passed
• 9 focused media resolver tests passed
• engine typecheck passed
• formatting and lint passed
• end-to-end fixture changed from video-only to video plus audio
• full video extractor suite had 79 passing tests and 2 unrelated failures because the environment FFmpeg lacks -fps_mode