refactor(control-plane): remove sketch_algebra layer

[zzylol]

Summary

• remove the top-level control_plane/src/sketch_algebra directory and module
• place backend-owned adapters under physical/post_asap
• rename the backend wrapper from L4Plan/physical_expr to PostAsapPlan/deployment_expr
• document that ASAPPlanner owns post-ASAP planning while the backend owns physical placement, deployment cost, emission, and runtime matching
• update the repost mock to cover the complete streaming/routing/BackendPlan publication contract

Verification

• cargo test -p control_plane --lib (688 passed)
• cargo test -p data_plane query_engines::asap_query_engine --lib (90 passed)
• cargo test -p data_plane --test e2e_controller_plans_and_backend_serves controller_really_pushes_backend_plan_and_query_really_serves (passed)
• git diff --check
• repository search confirms no sketch_algebra, L4Plan, or physical_expr references remain in control_plane/data_plane/README

Stack

Depends on #466.