F2.1b — Extract the shared ProjectFile schema boundary

[Flow-Fly]

Parent: #133. Grandparent: #132 / #72. Depends on: #270.

Goal

Create a shared, runtime-validatable ProjectFile contract that the existing client and future server can consume without moving the client or changing editor behavior.

Scope

• Add the owner-approved shared/ workspace from the ADR.
• Move or expose project schema types, version constants, and runtime validation there.
• Preserve existing client imports through small compatibility re-exports.
• Add round-trip and invalid-payload tests at the shared boundary.
• Keep current v4 migration behavior intact.

Out of scope

• No server process, database, object storage, auth, sync, or deployment.
• No broad client import rewrite.
• No new project format version unless a real schema change requires one and receives separate approval.

Acceptance

• The client builds and tests against the shared contract without behavior changes.
• A future server can validate a complete ProjectFile without importing client-only modules.
• Existing v4 files still load and round-trip.

Risk and routing

Risk: medium until the ADR and a fresh code audit are complete.
Worker-ready: no.
Review need: careful schema and migration review.