cost model: estimate network and materialization bytes at explicit boundaries #346

Follow-up to #323.

Scope

Model network transfer and materialization bytes when a selected physical plan crosses an explicit deployment, exchange, or persistence boundary. Do not charge logical in-memory edges as network or materialization traffic.

Acceptance criteria

• Represent explicit boundary kinds and their required evidence in the physical plan.
• Define dimensional formulas for network and materialization bytes, including execution multiplicity.
• Count shared transfers/materializations once and per-consumer transfers at their effective multiplicity.
• Fail closed for incomplete or incompatible boundary evidence.
• Preserve these dimensions separately until an explicit versioned calibration ranks them.
• Export totals, per-node terms, and provenance to dag_export and the viewer.