Add layout scan physical plan model

[joseph-isaacs]

Summary

• add a standalone Plan / PlanRef model for layout scan planning
• represent flat, chunked, dictionary, list, struct, expression, and row-index plans
• reject unsupported layout kinds instead of wrapping them in a generic layout plan
• lazily initialize and cache high-fanout chunk and struct-field plans
• preserve lazy children through optimization so unrequested descendants remain unmaterialized
• keep expression-plan construction separate from explicit plan optimization
• expose stable logical child ordering and composable plan-tree display
• construct built-in plan trees through vortex_layout::plan::new_plan
• add plan-only tests
• define a layout scan plan as the query-specific physical plan of layout reads needed to satisfy one query
• leave plan execution and its result API to a future PR

Checks

• cargo test -p vortex-layout --doc