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/surfaces.py

@@ -0,0 +1,75 @@
+"""Surface-feature executors (extrude_surface / revolve_surface).
+
+Surface features register an independent shell and never touch the active
+solid body's fuse/cut lifecycle.
+"""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING, Any
+
+from ..extents import _normal_from_sketch
+from ..registry import atomic_executor
+from ..specs import AxisSpec, PlaneSpec, vector_cross, vector_dot, vector_scale, vector_unit
+from ..topology import FeaturePlanNode, FeatureResult
+from .common import _revolve_axis, _validate_revolve_axis_in_sketch_plane
+
+if TYPE_CHECKING:  # pragma: no cover - import for type checkers only
+    from ..session import ExecutionSession
+
+
+def _execute_revolve_surface(node: FeaturePlanNode, session: "ExecutionSession") -> FeatureResult:
+    # Surface revolve 的 profile 是单一闭合 wire。它只生成独立 shell，不能参与
+    # 当前实体 body 的 fuse/cut，也不能把其结果误报为新的实体 body。
+    sketch = session.sketches.get(str(node.sketch_id))
+    if sketch is None:
+        raise ValueError("surface revolve has no resolved sketch")
+    faces = session.adapter.faces_for_sketch(sketch)
+    if len(faces) != 1 or faces[0].inner_wires():
+        raise ValueError("surface revolve requires exactly one closed profile without holes")
+    axis = _revolve_axis(node, session)
+    _validate_revolve_axis_in_sketch_plane(axis, sketch)
+    angle = float(node.params.get("angle_deg") or 0.0)
+    if angle <= 0:
+        raise ValueError("surface revolve requires angle_deg > 0")
+    if bool(node.params.get("reverse")):
+        angle = -angle
+    surface_id = session.register_surface(
+        node.feature_id,
+        session.adapter.revolve_surface(faces[0].outer_wire(), angle, axis),
+    )
+    return session.result(node, include_body=False, surface_id=surface_id)
+
+
+@atomic_executor("revolve_surface")
+def _revolve_surface_executor(node: FeaturePlanNode, session: "ExecutionSession", sketch: dict[str, Any] | None) -> FeatureResult:
+    del sketch
+    return _execute_revolve_surface(node, session)
+
+
+def _execute_extrude_surface(node: FeaturePlanNode, session: "ExecutionSession") -> FeatureResult:
+    # surfaceEntities 的曲面拉伸沿用实体特征已 lower 的距离，但始终独立登记为
+    # shell。它既不改变 active solid，也不以曲面参与实体 fuse/cut。
+    sketch = session.sketches.get(str(node.sketch_id))
+    if sketch is None:
+        raise ValueError("surface extrude has no resolved sketch")
+    direction = vector_unit(_normal_from_sketch(sketch), field_name="sketch normal")
+    if bool(node.params.get("reverse")):
+        direction = vector_scale(direction, -1)
+    distance = float(node.params.get("distance_mm") or 0.0)
+    if distance <= 0:
+        raise ValueError("surface extrude requires distance_mm > 0")
+    wires = session.adapter.surface_wires_for_sketch(sketch)
+    surface = session.adapter.extrude_surface(wires, vector_scale(direction, distance))
+    reverse_distance = float(node.params.get("reverse_distance_mm") or 0.0)
+    if reverse_distance > 0:
+        opposite = session.adapter.extrude_surface(wires, vector_scale(direction, -reverse_distance))
+        surface = session.adapter.combine_surfaces(surface, opposite)
+    surface_id = session.register_surface(node.feature_id, surface)
+    return session.result(node, include_body=False, surface_id=surface_id)
+
+
+@atomic_executor("extrude_surface")
+def _extrude_surface_executor(node: FeaturePlanNode, session: "ExecutionSession", sketch: dict[str, Any] | None) -> FeatureResult:
+    del sketch
+    return _execute_extrude_surface(node, session)