[codex] Ship Timbre Studio release pipeline

[Etherll]

What changed

• Adds Timbre Studio, a Tauri v2 desktop console for the Timbre pipeline.
• Adds the Studio GitHub Actions workflow for PR validation and signed release builds.
• Adds Studio docs, screenshots, manual regression checklists, and release/update guidance.
• Configures the Tauri updater and bumps Studio dev tooling to Vite 8 / Vitest 4 to clear the high-severity npm audit findings.

Release readiness

• Uploaded TAURI_SIGNING_PRIVATE_KEY to the repo Actions secrets from the ignored local updater key.
• Confirmed the committed updater public key matches the generated .pub file content.
• No private signing key or build output is committed.

Validation

• python -m pytest -q passed.
• Clean repo-shaped temp copy: npm ci passed.
• npm test passed: 22 tests.
• npm run build passed.
• cargo check --locked passed.
• npm audit --audit-level high passed: 0 vulnerabilities after the Vite/Vitest update.
• actionlint .github/workflows/studio-release.yml passed.

Not run

• Tier-2 manual GPU checklist; it requires a real 16 GB GPU box and remains the manual release sign-off gate documented in studio/docs/TIER2_CHECKLIST.md.