feat: compile Planner selections into physical DAG candidates

[zzylol]

Part 4 of the #462 split. Stacked on #474; it builds on #472, which is already merged into main.

Before this PR

The Planner selects logical Post-ASAP candidates (#472, merged) and has a shared runtime (#474), but nothing turns a selection into executable physical DAGs. Deployments do that lowering themselves.

After this PR

Logical Post-ASAP candidate + maintenance requirements
    → physical_planner compilation
    → precompute/query physical DAGs + typed materialization frontier
    → deployment: source/state binding, costing, storage
    → shared runtime (#474)

• Reader-independent compilation of selected candidates. Binding never repeats logical lowering.
• Bounded frontier enumeration and comparable workload-cost selection, including Rate → grouped Sum at both placements.
• Automatic temporal KLL pane compilation, and PromQL row/value lowering with dynamic-label vectors.
• The compiler emits typed readouts (feat: add native physical operators and shared DAG runtime #474): logical SketchQuery passes through unchanged, so there is no Statistic/string-parameter round trip.
• CompiledPhysicalDag and PhysicalCandidate derive serde and are validated on deserialization. They have no Planner byte format or version.
• dag is kept as a compatibility re-export.

Boundary change from #462

The Planner stays deployment-agnostic. These parts of #462 are left out and go to ASAPQuery-backend in a separate PR:

• the stored-definition semantic fragment and its dataset binding;
• SDS wire decoding and delta reconstruction, plus native batch frames;
• the versioned JSON envelopes for physical plans.

The design doc states this ownership.

Validation

cargo test --workspace passed 1356 tests, covering physical DAGs, plan recovery, precompute candidates and populations, PromQL values and binaries, weighted TopK binding, current-series heaps, and the KLL pane, SQL→physical and lifecycle end-to-end tests. Stored pane states are passed in memory; byte encodings are tested in sketchlib and deployments. cargo clippy --workspace --all-targets -D warnings is clean. These are shared-library tests, not deployment end-to-end tests.

Split stack: #472 · #473 · #474 · this PR.

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