refactor(logical-optimizer): move Stage 1's realization helpers out of the legacy search

[zzylol]

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

Rebased: added fix: integrate with #621: the executor's UnivMon test imports default_size_params from pass1::realization.

Problem

Q68 (a) deletes the legacy search (ASAPStrategies, search_workload*, legacy grouping/composition/propagation). Stage 1 still reached into it: pass1/logical_candidates.rs, pass2/summary_capability.rs, pass2/window_composition.rs and the accuracy estimators imported their realization catalogue and summary-input rules from pass1/replacement.rs, pass1/grouping.rs and pass2/reconciliation.rs. Deleting those modules would break the stage pipeline.

Changes

Pure move, no behaviour change:

• New pass1/realization.rs (Stage 1's realization catalogue and input rules), moved verbatim from the legacy modules:
  • from replacement.rs: Realization, DEFAULT_DELTA, summary_candidates, accuracy_target, accuracy_budget, default_size_params, PhysicalSummaryInput, PhysicalSummaryInputRuleResult, realize_keyed_additive_summary_input, schema_column_ref, summarised_column, column_ref, summarised_input
  • from grouping.rs: has_subpopulations (with its tests)
  • from reconciliation.rs: dominates
• hydra_guarantee (and its test) moves from grouping.rs to accuracy/estimators/mod.rs, its only Stage 1 caller.
• The one signature change the move needs: summarised_input returns Result<_, &'static str> instead of the legacy RealizationError; the legacy caller wraps it in RealizationError::PhysicalRealization, with the same message.
• Doc comments on the moved items no longer link to legacy functions.
• Callers repointed: the legacy modules, the crate re-exports (Realization, summary_candidates, has_subpopulations now come from pass1::realization), and the external paths in plan-selection's cost_model.rs / empirical_cost.rs and three tests (planner/tests/summary_sharing.rs, frontend-sql/tests/frequency_l2.rs, integration-tests/tests/planner_layering_example2.rs). No re-export stays at the old paths.

After this PR, the stage pipeline's modules (pass1/logical_candidates.rs, pass1/realization.rs, pass2/identical_expressions.rs, pass2/summary_capability.rs, pass2/window_composition.rs, accuracy/estimators/*) import nothing from pass1::{replacement, grouping, exact_composition, rollup, rewrite, explanation, maintained_population} or pass2::{reconciliation, topk_reuse} (checked by grep). accuracy/mod.rs still imports exact_composition::ExactOperation for AccuracyModel::exact_operation_rule. That trait method is dropped in step 8 (Q53).

Stacked on #630.

Test plan

• cargo fmt --all --check
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• cargo test --workspace --locked: 1519 passed, 0 failed, 14 ignored (after the rebase on main d4869a7; was 1495 passed, 22 ignored)

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