Port TS assignDashboardMainParents (orchestrator.ts:401-484) to Rust.
Implements §4.6: synthesizes fill_container Slot frames for single-subtask
rows, and horizontal Dashboard Row frames + proportional-width Slot frames
for multi-subtask rows (min 220 px per slot, last slot always fill_container,
available_width floor 320 px). Splits §4.6 into dashboard_columns/slots.rs
to keep all files under the 800-line ceiling; TDD test suite in
dashboard_columns_tests_b2.rs (10 tests, 298 total passing).
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