refactor(cdsl_engine): executor registry + per-family executor package

Phase 3 of the decoupling refactor (behavior-preserving):
- registry.py: atomic_executor decorator, ALL_ATOMIC_IDS with
  fail-fast registration validation, execute_node dispatcher
- executors/: one module per family (extrude, revolve, surfaces,
  loft_sweep, bodies, context, primitives, parametric, holes,
  dressup, patterns) + shared helpers in executors/common
- executors/__init__: explicit aggregation + completeness check
  (registry must cover every declared atomic id at import time)
- runtime.py: slimmed to entry points (analyze_cdsl/rebuild_cdsl)
  plus full historical re-exports incl. test-referenced privates

Adding an atomic operation now touches only one executor module and
its schema contract; the shared registry never changes. Verified
against baseline: zero new failures.

backend/engine/cdsl_engine/executors/__init__.py

@@ -0,0 +1,28 @@
+"""Executor modules for the session runtime.
+
+Importing this package registers every atomic executor into
+``cdsl_engine.registry.EXECUTORS`` exactly once, then verifies that the
+registry covers the complete declared atomic-id set.  Executor modules must
+not import each other; shared helpers live in ``common``.
+"""
+
+from __future__ import annotations
+
+from ..registry import ALL_ATOMIC_IDS, EXECUTORS
+from . import (  # noqa: F401  (importing the modules performs registration)
+    bodies,
+    context,
+    dressup,
+    extrude,
+    holes,
+    loft_sweep,
+    parametric,
+    patterns,
+    primitives,
+    revolve,
+    surfaces,
+)
+
+_missing = sorted(ALL_ATOMIC_IDS - set(EXECUTORS))
+if _missing:
+    raise RuntimeError(f"atomic ids without a registered executor: {_missing}")