Add recompute_pool_states + expire_cooling_pipelines + recompute_global_state
and PipelineEntry struct so warm pool entries carry warmth state + cooling
deadline. Drop the incomplete tuple-shape from the previous push.
- Cells own position directly (row/col/row_span/col_span)
- Drop regions JSON from layouts (cells ARE the regions)
- Drop is_default from layouts (display.default_layout_id owns)
- Drop grid_cols/grid_rows from layouts (computed from cells)
- Layout new form: name, description, priority, resets_idle_timer only
- Layout edit: visual grid builder, + buttons on cell edges,
click cell to assign content
- Bundle cells now carry position directly
- Rust kiosk attaches widgets using cell position
- Migration v0.4: backfills cell positions from old region map
