Persist and approve Studio diagram edit revisions

[anandpant]

Goal

Make browser edits in Studio durable: save an edited drawing as a new immutable revision and let the user explicitly approve a revision.

Current baseline

• Studio project and diagram records in packages/studio/projects point to one source artifact and do not model revisions or approval.
• apps/playground/src/routes/diagrams_/$diagramId/edit.tsx detects edits but only offers an Excalidraw download.
• Code Mode derived artifacts and the CLI revision store provide useful provenance and recovery precedents, but they are separate persistence systems and should not become a compatibility layer.

Scope

• Define a small Studio revision model around the existing Cloudflare object-store boundary.
• Persist an edited drawing as a derivative of its source artifact while keeping the source recoverable.
• Retain the source relationship, diagram type, content identity, and available generation metadata.
• Add an explicit approval action and record which revision was approved.
• Refactor the touched Studio edit data-loading lifecycle to TanStack Query or event-driven state; do not add or preserve direct useEffect synchronization in product code.
• Verify the real Studio edit/save/approve flow in a browser.

Non-goals

• Collaborative editing or event sourcing.
• Automatic prompt/model optimization.
• Reusing the CLI's local manifest format as Studio's storage contract.
• Community templates or publishing workflows.

Acceptance criteria

• A Studio edit can be saved and reloaded as a new immutable revision.
• The prior revision and source artifact remain recoverable.
• Approval is explicit, durable, and attached to one revision.
• Unit/integration coverage proves revision and provenance invariants.
• Browser coverage proves the current Studio edit flow can save, reload, and approve.

Current touchpoints

• apps/playground/src/routes/diagrams_/$diagramId/edit.tsx
• packages/studio/projects/src/contracts.ts
• packages/studio/projects/src/server/service.ts
• packages/diagram/agent/src/lib/code-mode-artifacts.ts
• apps/cli/src/storage.ts (precedent only)

This replaces the persistence-and-approval portion of #63 against the current repository.

[anandpant]

Follow-up: #283 will define consent-aware export into reviewable eval candidates after this issue establishes durable revision and approval identity.