refactor(planner): extract the Stage 3 plan-selection crate

[zzylol]

Rebased on main d4869a7 (DF 54).

Why

#572 puts each #509 stage in its own crate so that Cargo enforces the one-way stage flow. This PR is step B4: Stage 3 (plan selection, the only stage that uses the cost model) moves out of asap-aware-mapping into a new crate, asap-plan-selection. As #572 decided, three modules with no callers are deleted instead of moved. Behavior is otherwise unchanged.

What

Before this PR

frontends ──► asap-aware-mapping (Stage 3 + facade) ──┬──► asap-physical-optimizer (Stage 2) ──► asap-types
                                                      └──► asap-logical-optimizer (Stage 1) ──► asap-types
asap-aware-mapping/src/
  plan_selection/{mod, candidate_selection}  cost_model analytical_cost physical_plan_cost_model
  physical_operator_statistics physical_handoff_cost recurrence empirical_cost empirical_resources
  query_physical_lowering storage_io  pane_sharing erp empirical_comparison  pass/  test_support

After this PR

asap-planner ──► asap-aware-mapping (facade: pass/) ──► asap-plan-selection (Stage 3)
asap-plan-selection ──┬──► asap-physical-optimizer (Stage 2) ──► asap-types
                      └──► asap-logical-optimizer  (Stage 1) ──► asap-types
no path from asap-plan-selection to asap-aware-mapping, asap-planner or asap-physical-operators

plan-selection/src/
  lib.rs                 select_plan, select_exhaustive, PlanningModels   (was plan_selection/mod.rs)
  candidate_selection.rs legacy cost-ranked selection (deleted under #580)
  cost/                  cost_model analytical_cost physical_plan_cost_model physical_operator_statistics
                         physical_handoff_cost recurrence empirical_cost empirical_resources
                         query_physical_lowering storage_io
asap-aware-mapping/src/
  lib.rs  pass/{mod, stage_pipeline}

cargo tree -p asap-plan-selection --depth 1 -e normal,dev:

asap-plan-selection
├── asap-logical-optimizer
├── asap-physical-optimizer
├── asap-types
├── serde
├── serde_json
└── thiserror
[dev-dependencies]
└── asap-frontend-promql

How

• git mv for 16 files (table below). plan_selection/mod.rs becomes the crate root, so PlanningModels (Q37), select_plan and select_exhaustive are at asap_plan_selection::…. The crate root also keeps the re-exports asap-aware-mapping's root had for the moved items: CostModel, DefaultCostModel, the recurrence types and the candidate_selection types.
• Placement. Everything priced by Stage 3 goes under cost/, including query_physical_lowering and storage_io (Reorganize crates and modules by #509 stages and the #511 unified IR #572 update: only cost code calls them) and empirical_resources (used by empirical_cost). There is no accuracy/ module: its two planned members, erp and empirical_comparison, are deleted, and moving the AccuracyModel trait there (Q22) is a behavior change, not part of this move.
• No compatibility re-exports. A script, followed by cargo check, rewrote imports across the workspace: crate::cost_model → crate::cost::cost_model inside the crate, and asap_aware_mapping::{cost_model, …, plan_selection} → asap_plan_selection::… in asap-aware-mapping/pass, asap-physical-operators, devtools, integration-tests, planner, frontend-promql tests and the docs. pass no longer re-exports PlanningModels; asap-planner re-exports it from asap_plan_selection.
• Manifests. asap-aware-mapping drops serde, serde_json and its dev-dependency. devtools, integration-tests, frontend-promql (dev) and asap-physical-operators (dev) now depend on asap-plan-selection instead of asap-aware-mapping, which they no longer import. frontend-sql drops an unused asap-aware-mapping dev-dependency.
• Paths. include_str! paths in empirical_cost.rs gain one ../ for the deeper location.
• Manifest guards. A new test, tests/stage3_dependencies.rs, fails if the manifest names asap-aware-mapping, asap-planner or asap-physical-operators. The Stage 1 guard also rejects asap-plan-selection.
• Docs. Paths that cite moved modules are updated. offline-sketch-evidence.md and the ERP proposal now say that their consumers were deleted. The historical table in planner-vocabulary-migration.md is unchanged.

Deleted modules (no callers)

At the base of this PR, git grep -n -w for each module name and its public items (PaneReuseCandidate, SharedPaneGroup, select_shared_panes, ErpArtifact, ErpSelection, recommend_offline, OfflineComparisonRequest), outside the modules themselves, finds only the pub mod lines and documentation:

crates/asap-aware-mapping/src/lib.rs:41:pub mod empirical_comparison;
crates/asap-aware-mapping/src/lib.rs:44:pub mod erp;
crates/asap-aware-mapping/src/lib.rs:45:pub mod pane_sharing;
docs/design_docs/proposals/error-resource-profile.md:4,13   (ERP module link and status)
docs/develop_docs/offline-sketch-evidence.md:105            (`empirical_comparison::recommend_offline`)

Module	Lines	Tests deleted
pane_sharing.rs	113	2: shares_compatible_windows, excludes_invalid_and_nonbeneficial_quotes
erp.rs	859	10: selects_cheapest_applicable_accurate_configuration, distribution_mismatch_fails_closed, multi_fit_selects_only_confident_well_fitting_family, multi_fit_rejects_ambiguous_or_poor_observations, multi_fit_jointly_ranks_all_plausible_families, exact_empirical_fingerprint_precedes_fits_and_requires_identity, deserializes_sketch_bench_wire_format, nearest_shape_prefers_cardinality_and_skew_then_cost, nearest_shape_rejects_distribution_family_and_small_benchmarks, pane_window_composes_atomic_operation_costs
empirical_comparison.rs	915	9: accuracy_requirement_changes_selected_configuration_and_cost, preparation_and_exact_resource_dimensions_keep_their_meaning, optional_exact_dimensions_cannot_hide_invalid_measurements, sketch_comparison_requires_unknown_preparation_phase, missing_cost_or_failed_error_acceptance_selects_exact, applicability_is_checked_before_recommendation, resource_objective_and_unknown_memory_are_explicit, formal_minimums_cannot_be_relaxed_by_observed_accuracy, comparison_requires_disjoint_phases_and_one_exact_generation
empirical_cost::snapshot_prepare_cpu (pub(crate))	16	1: unmeasured_preparation_for_other_families_stays_unknown. Its only production caller was empirical_comparison (empirical_comparison.rs:414); without it the function is dead code and fails -D warnings.

Moved files

From asap-aware-mapping/	To plan-selection/
src/plan_selection/mod.rs	src/lib.rs
src/plan_selection/candidate_selection.rs	src/candidate_selection.rs
src/{cost_model, analytical_cost, physical_plan_cost_model, physical_operator_statistics, physical_handoff_cost, recurrence, empirical_cost, empirical_resources, query_physical_lowering, storage_io}.rs	src/cost/*.rs
src/test_support.rs	src/test_support.rs
tests/{physical_handoff_cost, storage_io}.rs, tests/data/offline-evidence-synthetic.json	tests/…

New: Cargo.toml, src/cost/mod.rs, tests/stage3_dependencies.rs.

Volume

62 files changed, +340 / −2,208. 16 files moved with git mv, 3 new, 3 modules (1,887 lines) deleted. About 170 changed lines name the new paths.

Gate (at the tip)

• cargo fmt --all --check: passes.
• cargo clippy --workspace --all-targets --all-features -- -D warnings: passes.
• cargo test --workspace: 1,508 passed, 12 ignored. That is B3's 1,529, less the 22 deleted tests listed above, plus the new manifest guard. Compared with refactor: extract the Stage 1 logical-optimizer crate (#572 B2) #583 (1,528): −22 + 2 guards.
• python3 -m unittest discover -s tools/dag-viewer -p test_render.py: OK, 6 skipped.
• stage_pipeline --example planner-layering-1 regenerates tools/dag-viewer/examples/planner-layering-example1.json byte for byte.

Part of #572 (B4) and #509. Follows #584. Related: #580 (deletes candidate_selection). Next: B4b (facade into asap-planner).

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