Excel model export & arbitrary-spreadsheet import

Export a builder network to .xlsx, or bring a network in from a DekEn-template spreadsheet or an arbitrary spreadsheet of someone else's making — using a column-mapping UI to tell DekEn which column means what.

• A team tracks pipe/pump/junction data in a spreadsheet before any model exists in a dedicated tool
• Bulk-editing many pipes' properties is faster in Excel than clicking through the builder
• Migrating a legacy spreadsheet-based design into a solvable DekEn network

The DekEn-template path maps 1:1 onto node/pipe fields; the arbitrary-spreadsheet path renders a mapping UI where you assign each source column to a target field before the import commits.