refactor(planner): extract the Stage 2 physical-optimizer 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 B3: Stage 2 (physical candidates) moves out of asap-aware-mapping into a new crate, asap-physical-optimizer. It is a pure move; behavior is unchanged.

What

Before this PR

frontends ──► asap-aware-mapping (Stage 2 + 3 + facade) ──► asap-logical-optimizer (Stage 1) ──► asap-types

asap-aware-mapping/src/
  physical_candidates  plan_selection/  cost_model recurrence analytical_cost …  pass/

After this PR

frontends ──► asap-aware-mapping (Stage 3 + facade) ──┬──► asap-physical-optimizer (Stage 2) ──► asap-types
                                                      └──► asap-logical-optimizer (Stage 1) ──► asap-types
asap-physical-optimizer ··dev··► asap-logical-optimizer, asap-frontend-promql   (tests only)

physical-optimizer/src/
  implementation/physical_candidates.rs
asap-aware-mapping/src/
  plan_selection/  cost_model recurrence analytical_cost … storage_io  pass/

cargo tree -p asap-physical-optimizer --depth 1 -e normal,dev:

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

How

• git mv asap-aware-mapping/src/physical_candidates.rs to physical-optimizer/src/implementation/physical_candidates.rs. materialization/ does not exist yet, so no empty module is created.
• Dependencies. Production code imports only asap-types: a Stage 1 candidate reaches Stage 2 as OperatorNode roots. asap-logical-optimizer is therefore a dev-dependency (the tests build candidates with enumerate_local_logical_candidates), not a normal one. The PromQL front end is also a dev-dependency.
• test_support. The crate has its own copy with only lower_promql.
• No compatibility re-exports. Callers now import asap_physical_optimizer::implementation::physical_candidates::…: plan_selection/mod.rs (two sites) and integration-tests/tests/planner_layering_example1.rs (two sites). asap-aware-mapping and integration-tests gain the dependency.
• Manifest guards. A new test, tests/stage2_dependencies.rs, fails if the manifest names asap-plan-selection, asap-aware-mapping or asap-physical-operators. The Stage 1 guard also rejects asap-physical-optimizer now.

Moved files

From	To
asap-aware-mapping/src/physical_candidates.rs	physical-optimizer/src/implementation/physical_candidates.rs (unchanged, 281 lines)

New: Cargo.toml, lib.rs, implementation/mod.rs, test_support.rs and tests/stage2_dependencies.rs.

Volume

14 files changed, +143 / −15. One file moved, 5 new, 4 import sites rewritten.

Gate (at the tip)

• cargo fmt --all --check: passes.
• cargo clippy --workspace --all-targets --all-features -- -D warnings: passes.
• cargo test --workspace: 1,529 passed, 12 ignored. That is refactor: extract the Stage 1 logical-optimizer crate (#572 B2) #583's 1,528 plus the new manifest guard. No test is deleted.
• 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 (B3) and #509. Follows #583. Related: #580. Next: B4 (asap-plan-selection).

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