refactor: move AccuracyModel to plan-selection

[zzylol]

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

Problem

Q53: only Stage 3 checks an estimate against its accuracy target, but the AccuracyModel trait and DefaultAccuracyModel lived in Stage 1's crate. The legacy search was the reason; #635 deleted it.

Changes

• New asap_plan_selection::accuracy module, re-exported at the crate root:
  • AccuracyModel with local_guarantee and satisfies.
  • DefaultAccuracyModel, which delegates to logical-optimizer's estimators.
  • PlanningModels now takes this trait. As before, Stage 3 rejects an estimate whose family has no model (local_guarantee returns None).
• asap_logical_optimizer::accuracy no longer defines a model. It exposes the analytical functions the default delegates to:
  • local_guarantee and sketch_guarantee (made pub)
  • satisfies, the conservative target check moved out of DefaultAccuracyModel::satisfies unchanged
  • The estimator unit tests call these functions directly. The crate root no longer re-exports AccuracyModel / DefaultAccuracyModel.
• propagate and exact_operation_rule were already dropped in chore: delete the legacy replacement search and CostModel #635 (they depended on deleted modules).
• Test models repointed to asap_plan_selection::{AccuracyModel, DefaultAccuracyModel}: planner/tests/summary_sharing.rs (UnivMonEvidence) and frontend-promql/tests/univmon_candidates.rs.
• The stage_pipeline devtool needs no change: it reports the model by name ("DefaultAccuracyModel").
• The facade re-exports only PlanningModels, so it needs no change either.

Rebase note: rebased onto #627: the plan-selection AccuracyModel keeps answers(statistic, guarantee) next to satisfies, as a default method delegating to the new asap_logical_optimizer::accuracy::answers, and DefaultAccuracyModel delegates it like the other two methods; Stage 3 still calls models.accuracy.answers(...). #621's UnivMon L2 tests now call local_guarantee/satisfies directly.

Stacked on #635.

Test plan

• 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)

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