Open Personal-app-generated diagrams in the Pro app with correct layout.

Done condition: Pro app opens a Personal-generated diagram with correct auto-layout, no data corruption, no FR-2 violation.

Current status: Blocked on Goal 2. Avoid risky Pro app changes until Goals 1+2 validated.

Risks:

• Auto-arrange (T2-1) is 2-3 weeks of work — consider simpler generational Y-alignment as interim solution
• FR-2 violation (T0-4) only triggers on concurrent Pro+Personal writes; low priority until Goal 2 is live