[zzylol]

Part of #879

Why

asap-plan-selection grew three searches (the current whole-workload search, the memo brute force and the memo MILP) and the memo's per-node pricing under one pass/subexpr/ directory, next to cost files at the crate root. This is step 1 of the approved layout: a search/ tree by search algorithm and a cost/ tree for everything that prices. Moving the files first, with no logic change, keeps the later splits (of lib.rs and milp.rs) reviewable as small diffs.

What

Files move into search/legacy, search/memo (with rules/), search/milp and cost/. Module and use paths are updated. Nothing else changes: no logic, no item renames, and the crate root's public paths stay the same.

How

• git mv per the table below, plus three small new mod.rs files (search/legacy, search/memo/rules, search/milp).
• Relative super:: paths in the moved files are rewritten as crate:: paths.
• Items that now cross a module boundary they did not cross before are pub(crate). That is every pub(super) in the moved files, plus a few private helpers that sibling modules used through super:: (in brute_force, per_node, measured).
• #![allow(dead_code)] is added to cost/per_node.rs and search/milp/mod.rs. They used to inherit it from the memo module.
• lib.rs declares pub mod search; with pub use search as pass; and pub use search::legacy::pane_sharing;, so asap_plan_selection::pass::* and asap_plan_selection::pane_sharing resolve as before and no other crate changes.
• Doc links and file paths in docs point at the new files.

Before	After
pass/mod.rs	search/mod.rs
pass/stage1_to_stage3.rs	search/legacy/stage1_to_stage3.rs
pass/components.rs	search/legacy/components.rs
pass/interchangeable.rs	search/legacy/interchangeable.rs
pass/query_candidates.rs	search/legacy/query_candidates.rs
pane_sharing.rs	search/legacy/pane_sharing.rs
pass/subexpr/mod.rs	search/memo/mod.rs
pass/subexpr/groups.rs	search/memo/groups.rs
pass/subexpr/capability.rs	search/memo/rules/capability.rs
pass/subexpr/windows.rs	search/memo/rules/windows.rs
pass/subexpr/scans.rs	search/memo/rules/scans.rs
pass/subexpr/search.rs	search/memo/brute_force.rs
pass/subexpr/milp.rs	search/milp/model.rs
pass/subexpr/coefficients.rs	search/milp/coefficients.rs
pass/subexpr/pricing.rs	cost/per_node.rs
measured_operation_costs.rs	cost/measured.rs
cost/analytical_cost.rs	cost/analytical.rs
cost/physical_operator_statistics.rs	cost/operator_statistics.rs

All paths are under crates/plan-selection/src/.

Key code interfaces added / modified

None (paths only). Public paths kept: asap_plan_selection::pass::* (now an alias of search), asap_plan_selection::pane_sharing, and every crate-root re-export. The public cost submodules are renamed with their files (cost::analytical, cost::operator_statistics); no other crate uses them.

Major algorithm changes

None.

Before this PR

The memo search, its MILP and its per-node pricing lived together in pass/subexpr/; cost files were split between the crate root and cost/.

After this PR

Each search lives in its own directory under search/, and everything that prices lives under cost/. Splitting lib.rs and milp.rs comes in a later PR.

Verification

• cargo test -p asap-plan-selection: all tests pass.
• cargo check --workspace --tests: dependent crates build unchanged.
• cargo fmt --check and cargo clippy -p asap-plan-selection --all-targets -- -D warnings: clean.
• git diff -M --stat: every file is detected as a rename. The remaining line changes are use and module paths.
• cargo doc -p asap-plan-selection --document-private-items: the same unresolved links as before the move, none new.