Add group_rollup_mode

[texodus]

• Breaking Replaces the per-query leaves_only parameter (on to_columns/to_rows/to_ndjson) with a new group_rollup_mode property of ViewConfig, making leaf-only behavior a first-class view setting rather than a serialization option.
• Adds a GroupRollupMode enum with "rollup" and "flat" variants to the protobuf schema and Rust client config, with a commented-out Total variant for future use.
• Refactors C++ engine traversal logic — moves row filtering from view.cpp into traversal.cpp and tree_context_common.cpp, so contexts handle rollup/flat modes natively.
• Adds UI support, a group_rollup_mode dropdown selector in the config panel., with corresponding LESS styling.
• Adds plugin metadata support (group_rollups) for datagrid and d3fc plugins, including datagrid rendering changes for flat mode (hiding tree expanders, adjusting column headers).

Screen.Recording.2026-03-03.at.11.31.13.PM.mov