bug: DataGrid2 column names replaced by sequential identifiers in DESCRIBE

[engalar]

DataGrid2 columns defined with semantic names are replaced by sequential identifiers (column1, column2) in DESCRIBE PAGE output. Column identity is lost; cannot diff or re-import DataGrid2 pages by name. Affects 03-page-examples.mdl. Severity: MEDIUM. Discovered via automated roundtrip test (2026-03-25).