chore(sketch_algebra): re-layer three cost/planning modules out of L4 IR

[zzylol]

Summary

Stage 4 of the sketch_algebra re-layering (Stage 1: ASAPController#141, Stage 2: #396, Stage 3: #397) — scoped down after a fresh audit found two of the originally-planned four items rested on a stale premise:

• sketch_params.rs has 14 real production consumers (optimizer/cost/wire.rs, optimizer/engine.rs, optimizer/rules/mod.rs, replan.rs, all of emit/*.rs, all of physical/colored_dag/*.rs), not "one real consumer" as the design doc claimed.
• PhysicalExpr::RawAtEdgeSketchAtBackend/RawAtEdgePrometheusArchive are actively constructed and matched across optimizer/rules/mod.rs, emit/{mod,telegraf,otap,stage_config}.rs, physical/colored_dag/{emitter,allocator}.rs, and referenced from data_plane/asap_types — not dead code. The "duplicate dead BindMode" claim was also wrong: optimizer::cost::wire::BindMode is a single, real, heavily-tested enum that deliberately mirrors PhysicalExpr's variant names.

Both dropped from this stage; a proper investigation into replacing sketch_params.rs with asap_sketch::SummaryKind/SummaryParams is separate future work given its real blast radius.

Three items whose premises held up:

1. Deleted sketch_algebra/schema.rs. SketchStateSchema/SketchStateMetadata had zero real callers — every hit was a stale doc-comment cross-reference.
2. Moved SketchCapability/SupportedIntent/default_capability_table/load_capability_overrides from sketch_algebra::capability into optimizer::cost::sketch_capability — this is the perf/cost-model half of the "four overlapping capability tables" Step 2a originally consolidated, read by the optimizer/physical planner, not L4 IR. sketch_algebra::capability keeps the query-side Capability/SketchKindHandle tag and capability_for, which are genuinely L4-adjacent.
3. Moved sketch_selection.rs to a top-level control_plane::sketch_selection module — its one external caller is query_planning.rs; naming candidate sketch families for a Capability is a query-planning concern, not L4 IR.

No behavior change — all three moves are verbatim relocations with updated doc comments and import paths.

Test plan

• cargo build --workspace — clean
• cargo test -p control_plane — 824 passed, 1 pre-existing unrelated failure (invalid_sketch_type_override_falls_back_to_default)

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