[LAY-001] Boundary-aware shared layout

[Hacks4Snacks]

Roadmap priority: P3

Goal: Provide deterministic coarse placement without changing trust-boundary semantics.

• Do not expose the current DiagramLayout through API/WASM unchanged.
• Add boundary membership constraints or move boundaries and members as a unit.
• Prevent component/component, component/boundary-title, and member/boundary-edge overlap; do not
  clip elements placed through --boundary.
• Prove boundary-crossing sets are unchanged before/after layout unless explicitly requested.
• Decide whether engine placement replaces only Studio Tidy's coarse placement while client-side
  sizing, routing, and label deconfliction remain.
• Offer one coherent Arrange/Tidy workflow.

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