docs(planner): document lifecycle-aware planning and costs - #320

…ase boundaries (#171)

Add phase-explicit post-ASAP nodes SummaryExpr::{ExactTransform, ExactPostProcess}
carrying a non-exhaustive ExactOperator::Aggregate payload (never an intact
QueryExpr subtree), plus an ExecutionAvailability {UpdateValue, SummaryState,
ReadoutValue} derivation/validation (post_asap::phase) returning typed
PhaseErrors at construction. construct_summary_agg now validates its edge, so
a maintained summary over a query-time readout falls back conservatively
instead of producing an unexecutable plan.

Add ExactCompositionStrategy (registered in default_strategies) proposing
Replacement::ExactComposition candidates that reference the child target
rather than selecting a child; PlanSpace::global_selection commits the
compatible parent/child pair using the issue's cost-units-per-second
formulas (postprocess/pretransform vs raw-recompute baseline), counts shared
child state once, and GlobalSelection::materialize links the committed
decisions into one validated DAG with shared Rc identity.

Cost hooks: CostModel::mixed_execution_capabilities and
exact_composition_cost_inputs (unknowns stay None, never zero; missing
statistics keep KeepPreAsap). DAG export gains explicit per-node stage,
decision provenance, cost unit and child-decision links (additive).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>