fix(processor): isolate loudnorm capture to graph finalisation

[flexiondotorg]

• Capture loudnorm JSON only while freeing the filter graph, where FFmpeg emits it.
• Avoid starting process-global log capture until graph teardown in Pass 3 and Pass 4.
• Tighten error handling so setup, loop, and encoder failures do not start capture.
• Expand tests for malformed JSON, missing JSON, serialised capture, and clean-up order.

[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Requires human review: Refactors global FFmpeg log capture and graph finalization logic; touches process-wide state and core audio processing paths.}