feat(studio): gate the VST FX section behind STUDIO_VST_ENABLED

[vanceingalls]

What

Gates the VST FX section and everything that triggers the sidecar behind a new STUDIO_VST_ENABLED flag (default off, VITE_STUDIO_ENABLE_VST=true to turn on), alongside the existing STUDIO_*_ENABLED flags in manualEditingAvailability.ts:

• PropertyPanel.tsx: the VstSection render, so the FX panel section never mounts for an audio element while the flag is off.
• useVstPreview.ts: the "lazily start the sidecar once a vst-chain track appears" effect — the sole call site of useVstHost's ensureStarted() in the app, so gating it here is sufficient (the FX panel never calls it directly, and useVstHost() itself does nothing until asked).

Left unflagged on purpose: engine's render-time VST chain application and the Lambda VST-chain rejection guard — those operate on whatever data-vst-chain content already exists in a composition's HTML regardless of which UI authored it, so gating them would silently change render behavior for existing content rather than just hiding an editing surface.

Why

New, unreleased studio surface — stage it in behind a flag for incremental rollout/review rather than shipping it live to everyone by default.

Test plan

• New tests proving VstSection is absent by default and present when the flag is on
• useVstPreview.test.tsx's existing 27 tests updated to mock the flag on, so they keep exercising the real behavior
• Full studio suite (2752 tests) + build + fallow gate all pass

🤖 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 👈 (View in Graphite)
• 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
• 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Superseded by a recut stack: #2919 → #2920 → #2921 → #2922 → #2923.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.