refactor(cdsl_engine): extract session/extents/pattern_transform from runtime

Phase 2 of the decoupling refactor (behavior-preserving move):
- runtime_base.py: RuntimeExecutionError, FeatureExecutionError, ExtentVector
- session.py: GeometryAdapter protocol + ExecutionSession
- extents.py: end-condition planning (_extent_vectors family)
- pattern_transform.py: translate/mirror/rotate replay parameter algebra
- runtime.py: keeps executors + registry + entry points; re-exports all
  moved names (incl. test-referenced privates) for import stability

No behavior change; verified against baseline (zero new failures).

backend/engine/cdsl_engine/runtime_base.py

@@ -0,0 +1,46 @@
+"""Shared runtime error types and extent planning values.
+
+These primitives sit below both ``session`` (execution state) and the
+executor modules, so geometry-independent helpers can raise the same stable
+execution errors without importing the session or any executor.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Any
+
+from .topology import RuntimeDiagnostic, Vector3
+
+
+class RuntimeExecutionError(RuntimeError):
+    """A feature execution failure with serializable runtime evidence."""
+
+    def __init__(self, diagnostic: RuntimeDiagnostic, selector_resolutions: list[dict[str, Any]]) -> None:
+        super().__init__(diagnostic.message)
+        self.diagnostic = diagnostic
+        self.selector_resolutions = selector_resolutions
+
+
+class FeatureExecutionError(RuntimeError):
+    """An expected feature-level execution rejection with a stable code."""
+
+    def __init__(self, code: str, message: str, **detail: Any) -> None:
+        super().__init__(message)
+        self.code = code
+        self.detail = detail
+
+
+@dataclass(frozen=True)
+class ExtentVector:
+    """Single-directional extrusion displacement for one profile face.
+
+    ``trim_to`` stays ``None`` for an exact vector extrusion.  When set, the
+    extent means "extrude until the target face, trimming any profile region
+    that does not reach it" (up_to_surface trim semantics, issue #5).  The
+    piercing distance is computed inside the adapter, so ``vector`` only
+    supplies the direction.
+    """
+
+    vector: Vector3
+    trim_to: Any | None = None