One workload-level PhysicalASAPDAG and PlanOutput deduplication — after #511

[zzylol]

Goal

#509 defines a candidate as a DAG for the whole workload, and selection returns one PhysicalASAPDAG with each shared summary costed once. Today:

• PlanOutput holds one QueryLifecyclePlan per query.
• PhysicalASAPDAG (renamed in refactor!: rename planner candidate types to the #509 stage names #514) is compiled from one root. A summary shared by feat: share identical summary producers across queries after Pass 1 #515 appears in several plans, and consumers dedupe it by Rc pointer.
• For a root that reaches a shared state, assembly's raw-vs-summary check (feat: share identical summary producers across queries after Pass 1 #515) compares that root's raw cost over the union's reads with the whole shared state. This is approximate unless the members cost about the same per read.
• Planning and selection of a candidate run per target group rather than as one whole-workload candidate costed as a unit. CandidateDagInventory / enumerate_candidate_dags exist, but only tests use them.

Blocked on

Implementation of #511 (unified operator graph: OperatorNode with common metadata, no KeepPreAsap wrappers). Do not start before #511 is implemented.
The merged per-workload graph should be built on #511's common operator graph rather than on today's per-root post-ASAP DAG.

Scope

1. Assemble the selected per-root plans into one workload DAG with shared nodes deduplicated, and compile that DAG into one PhysicalASAPDAG.
2. PlanOutput carries the workload-level plan, each query's root ID, and deduplicated deployments.
3. Replace feat: share identical summary producers across queries after Pass 1 #515's approximate shared-root raw-vs-summary check with a single whole-workload comparison.
4. Decide whether selection costs complete whole-workload candidates directly, or keeps per-group selection with class-level sharing (feat: share identical summary producers across queries after Pass 1 #515).

References

#509 (Stages, "3. Plan selection"), #514, #515, #519.