refactor(cdsl_engine): freeze legacy engine paths under legacy/

Phase 7 of the decoupling refactor (behavior-preserving move):
- legacy/llm_compiler.py, legacy/llm_engine.py: frozen build_pack path
  (unused by run_cdsl_only), moved with git mv for history
- legacy/exact_rebuild.py: _run_parameterized / _run_exact /
  _apply_geometric_compensations moved out of rebuild.py, including
  the part-specific compensation table
- rebuild.py keeps run_rebuild / run_cdsl_only / compare_with_gold and
  imports the moved functions
- llm_compiler.py / llm_engine.py become compatibility shims

Frozen zone: new capability belongs in executors/ + schema contracts.
The project-specific compensation no longer sits in the main pipeline.

backend/engine/cdsl_engine/legacy/__init__.py

@@ -0,0 +1,13 @@
+"""Frozen legacy engine paths.
+
+``llm_compiler`` (thin CDSL to build_pack) and ``llm_engine`` (build_pack
+executor) predate the session-based runtime and are not used by
+``run_cdsl_only``.  ``exact_rebuild`` hosts the SolidWorks-exact fallback
+rebuilds and the project-specific geometric compensations.
+
+Do not extend these modules; new capability belongs in the session runtime
+(``executors/``) or the schema contracts.  The top-level ``llm_compiler.py``
+and ``llm_engine.py`` shims keep every historical import path working.
+"""
+
+from __future__ import annotations