fix(export): keep mic sidecar in native audio mix

[meiiie]

Summary

• Keep companion mic/system sidecar audio in the native/static export audio plan when the source MP4 also has an embedded audio track.
• Thread the resolved fallback path list into offline audio rendering so the edited-track mix uses both embedded source audio and external sidecars.
• Add regression coverage for the fix(export): route Windows auto through native GPU planner #504-style case where the native exporter previously saw only the sidecar fallback.

Why

A latest beta user reported that a one-minute recording exported with voice suddenly gone. This matches a #504 audio-continuity slice: when native/static export selects an offline audio render, the exporter can drop the mic sidecar if it only treats the embedded source track as the audio source.

This PR is intentionally narrow. It does not claim to fix capture-path failures where the mic sidecar itself is blank or missing.

Verification

• npm test -- src/lib/exporter/modernVideoExporter.nativeStaticLayout.test.ts src/lib/exporter/sourceAudioFallback.test.ts src/lib/exporter/sourceTrackRoutingPolicy.test.ts electron/ipc/recording/diagnostics.test.ts
• npx tsc --noEmit
• npx biome check --formatter-enabled=false src/lib/exporter/modernVideoExporter.ts src/lib/exporter/modernVideoExporter.nativeStaticLayout.test.ts src/lib/exporter/sourceAudioFallback.ts src/lib/exporter/sourceTrackRoutingPolicy.ts electron/ipc/recording/diagnostics.ts
• git diff --check

Summary by CodeRabbit

• Tests

  • Added test coverage for companion audio (microphone) mixing with native video exports when source MP4 already contains audio.

• Improvements

  • Enhanced audio fallback path resolution for native video exports to properly handle mixed companion audio scenarios.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 8cfc522 and bc2b6d7.

📒 Files selected for processing (2)

• src/lib/exporter/modernVideoExporter.nativeStaticLayout.test.ts
• src/lib/exporter/modernVideoExporter.ts

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📝 Walkthrough

Walkthrough

ModernVideoExporter centralizes source audio fallback paths resolution for native exports by introducing a getNativeAudioFallbackPaths() helper that computes fallback paths by filtering configured sources and resolving additional external paths when videos lack embedded audio. This computed value threads through buildNativeAudioPlan() into the NativeAudioPlan type and flows downstream through three audio rendering call sites instead of relying directly on config.

Changes

Native Audio Fallback Paths Threading

Layer / File(s)	Summary
Type contract and import src/lib/exporter/modernVideoExporter.ts	NativeAudioPlan type is extended with optional sourceAudioFallbackPaths, and resolveSourceAudioFallbackPaths import is added to support path computation.
Audio fallback paths helper src/lib/exporter/modernVideoExporter.ts	New getNativeAudioFallbackPaths(videoInfo) helper filters configured fallback paths, optionally resolves external fallback paths when video lacks embedded audio and local source is available, and returns the augmented or original path list.
Audio plan building and fallback computation src/lib/exporter/modernVideoExporter.ts	buildNativeAudioPlan() calls getNativeAudioFallbackPaths() to compute sourceAudioFallbackPaths and threads the result into the returned offline-render-fallback plan shape for both primary and fallback branches.
Audio rendering integration src/lib/exporter/modernVideoExporter.ts	renderEditedAudioForNativeMux() implementation and three call sites (getNativeStaticLayoutAudioOptions, finishNativeVideoExport, finalizeExportWithFfmpegAudio) now pass sourceAudioFallbackPaths from the plan to renderEditedAudioTrack instead of using config directly.
Test coverage src/lib/exporter/modernVideoExporter.nativeStaticLayout.test.ts	New test verifies companion sidecar audio mixing when source MP4 contains audio, asserting both normalized MP4 and mic paths appear in sourceAudioFallbackPaths. Updated existing test adds sourceAudioFallbackPaths assertion to strengthen offline-render plan validation.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Possibly related PRs

• webadderallorg/Recordly#309: Updates ModernVideoExporter's native audio planning to compute and thread sourceAudioFallbackPaths via resolveSourceAudioFallbackPaths into FFmpeg/native audio rendering, directly matching this PR's downstream decoding behavior for embedded vs sidecar audio.
• webadderallorg/Recordly#297: Both PRs touch edited-track native audio planning; the main PR adds/threads sourceAudioFallbackPaths into buildNativeAudioPlan for offline-render-fallback while #297 introduces the filtergraph-fast-path vs offline-render-fallback split based on sourceAudioFallbackPaths availability.
• webadderallorg/Recordly#280: Both PRs propagate mic companion sidecar audio into fallback audio paths for export (via sourceAudioFallbackPaths in ModernVideoExporter offline-render-fallback muxing).

Suggested labels

Checked

Poem

🐰 hops excitedly
Audio paths now thread so fine,
From helper functions through the line,
Companion sidecars mix with cheer,
Fallback renders standing clear! 🎵

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: ensuring mic sidecar audio is preserved in the native audio export mix, which directly aligns with the primary objective.
Description check	✅ Passed	The description includes Summary, Why (motivation), and Verification sections covering all essential information, though it omits the formal template structure with checkboxes and Type of Change field.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/voice-audio-continuity

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