docs: describe the stage pipeline instead of the removed legacy search

[zzylol]

Stack: #574 → #620 → #618 → #621 → #627 → #625 → #628 → #632 → #634 → #616 → #617 → #622 → #624 → #629 → #630 → #631 → #633 → #635 → #636 → #637

Problem

After #630–#636, many docs still described APIs that no longer exist: the legacy search (ASAPStrategies, search_workload, CandidateLogicalASAPDAGs, cost_sorted, global_selection), CostModel / empirical_cost / PhysicalPlanCostModel / PhysicalEvidenceSnapshot, explain_replacements, dag_export, accuracy propagation, and the old sketch_coverage metrics.

Changes

These docs now describe the current API:

• develop_docs/library-api.md: rewritten around the stage pipeline. It covers lowering (unchanged), e2e_plan, plan_stages, Stage 1 inventory (stage1_logical_candidates, compose_logical_candidate), PlanningModels (accuracy, calibration, capabilities, evidence) and export (compile_logical_asap_dag, compile_physical_asap_dag). The legacy ranking, strategy and selection recipes are removed.
• develop_docs/asap-aware-mapping-architecture.md: replaced by a short description of Stages 1–3, where each concern lives, and how to add a realization.
• develop_docs/end-to-end-accuracy-guarantees.md: the candidate flow and location table now describe Stage 3's per-estimate check, and the trait snippet shows the plan-selection AccuracyModel. The composition-contract sections are marked as the removed legacy rules (Coverage lost with the legacy candidate_selection tests (follow-up of #580) #623).
• develop_docs/storage-operation-costs.md and physical-handoff-costs.md: the estimators price deployment-supplied profiles. The PhysicalEvidenceSnapshot adapter that fed them is gone.
• develop_docs/metrics-observability-corpora.md: describes the Stage 1 measurement from test: port the legacy-search callers outside logical-optimizer to Stage 1 #633.
• develop_docs/offline-sketch-evidence.md: status note that the planner-side consumer (empirical_cost) was removed; the artifact format is kept.
• develop_docs/local-logical-candidates.md: drops "the legacy search remains".
• design_docs/architecture/README.md and concepts/planner-pipeline.md: component flow and output are the stage pipeline and the selected plan.
• design_docs/architecture/input-output-workflow.md: status note at the top. The planning-model inputs rows now show PlanningModels. The output and workflow sections stay as a historical record.
• design_docs/concepts/accuracy-models.md (sizing paragraph), concepts/post-asap-ir.md (retain_exact), proposals/asap-aware-mapping/maintained-populations.md: one-line factual fixes.
• user_guide_docs/run-a-query.md: intro and sketch_coverage description.
• Repository README.md, docs/README.md and develop_docs/README.md: reading paths no longer send extenders to the legacy guides. Those guides are listed under "historical records".

Marked historical, rationale left unchanged:

• Banners: develop_docs/asap-aware-mapping-contracts.md, extend-asap-aware-mapping.md, replacement-explanations.md, target-candidate-api-migration.md, planner-vocabulary-migration.md; design_docs/architecture/asap-aware-mapping.md, asap-aware-plan-search.md, evidence-dependent-candidates.md; proposals/asap-aware-mapping/ddsketch-quantile-ratios.md.
• Status-line notes: decisions/cse-cost-model.md, decisions/concat-unique-keys.md, proposals/asap-aware-mapping/end-to-end-accuracy-guarantees.md, analytical-resource-cost.md, summary-properties.md, proposals/asapquery-rule-coverage.md, and architecture/updated_interface_with_pluggable_optimization.md.

Not changed: proposals/planner-layering.md uses CandidateLogicalASAPDAGs as the conceptual name of Stage 1's output, not the deleted Rust type. Broken source links remain only in documents marked historical.

Stacked on #636.

Test plan

Docs only.

• cargo fmt --all --check
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• cargo test --workspace --locked: 1280 passed, 0 failed, 14 ignored (after the rebase on main d4869a7; was 1256 passed, 22 ignored)
• grep for the deleted API names under docs/ finds them only in documents marked historical or removed; anchors into the rewritten library-api.md point at existing sections

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