refactor(vst): split oversized files and dedupe CLI/engine code to clear fallow gate

[vanceingalls]

What

Splits the oversized files this branch added or grew past the repo's 600-line filesize cap, and dedupes code the repo's fallow gate flagged as newly-introduced duplication/complexity:

• propertyPanelVstSection.tsx (816→429 lines) → propertyPanelVstShared.ts, propertyPanelVstCarveSection.tsx, propertyPanelVstPluginRows.tsx
• useVstHost.ts (673→356 lines) → vstHostWire.ts (protocol parsing, pending-request bookkeeping, socket connect sequence)
• useVstPreview.ts (806→397 lines) → useVstPreviewHelpers.ts (load-scan orchestration, track lifecycle, PCM decode)
• Decomposes timelineIframeHelpers.ts's flagged autoHealMissingCompositionIds and buildMissingCompositionElements
• Dedupes parseIntFlag/parsePositiveInt/parseEnum + the sites create verb guard shared by lambda.ts/cloudrun.ts into utils/distributedRenderFlags.ts
• Dedupes the ffmpeg-timeout/output-dir/result-building boilerplate shared by audioMixer.ts's extract/prepare/silence helpers
• Extracts shared test-render boilerplate (testRenderUtils.tsx) across the propertyPanelFlat*.test.tsx family, plus per-file test fixtures (vstSidecarTestFixture.ts, vstSocketTestFixture.ts) elsewhere

Why

All of the above is pure extraction/dedup with no behavior change, needed to clear the repo's fallow static-analysis gate (complexity/duplication/filesize) before this branch could land.

Test plan

• fallow audit passes clean (0 introduced complexity/duplication/dead-code findings)
• Full monorepo build succeeds
• Full test suite passes across studio/engine/cli/vst-host

🤖 Generated with Claude Code

[vanceingalls]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat(studio): gate the VST FX section behind STUDIO_VST_ENABLED #2605
• chore(vst-host): delete monorepo package, repoint at published PyPI package #2604
• refactor(vst): split oversized files and dedupe CLI/engine code to clear fallow gate #2603 👈 (View in Graphite)
• feat(vst): voiceover spectral-carve VST pipeline (V1) #2602
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[vanceingalls]

Superseded by a recut stack: #2919 → #2920 → #2921 → #2922 → #2923.

Same net result (68 files, +8,632/-491) with the add-then-delete churn removed and rebased onto current main (this stack's merge-base was 519 commits behind). Leaving these open until the new stack is reviewed.

Per-PR deletions now: 122 (a net-negative refactor), 1, 297, 15, 54 — all real edits against main, none of it code an earlier PR in the stack had added.

[vanceingalls]

Closing in favor of the recut stack: #2919 → #2920 → #2921 → #2922 → #2923.

Same net result (68 files, +8,632/-491), rebased onto current main, with the add-then-delete churn removed — packages/vst-host/ is never added and then deleted, files arrive pre-split, and the feature flag lands with the first studio commit instead of trailing the stack.

Branches are left in place; nothing is deleted.