fix(convert): IFC->STEP falls back to OCC when native coverage is incomplete (audit regression)

The native .ifc->step registration overrode OCC with NO fallback, so audit IFC files with
IfcExtrudedAreaSolid/CSG/tessellated geometry (or non-proxy elements) failed. native_ifc_to_step now
raises on incomplete coverage (products_skipped>0 / faces_dropped>0 / 0 solids); _via_ifc_to_step
catches it and uses the OCC writer (_via_ada_to_step). Verified on the IFC fixtures: advanced-brep ->
native, swept/CSG/non-proxy -> OCC fallback, none fail (one fixture fails inside OCC itself = pre-existing).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: Krande

src/ada/cadit/step/native_ifc_to_step.py

@@ -44,7 +44,14 @@ def native_ifc_to_step(
     stats = adacpp.cad.stream_ifc_to_step(str(ifc_path), str(out_path), deflection=deflection, angular_deg=angular_deg)
     if not stats or stats.get("solids_out", 0) <= 0:
         raise RuntimeError(f"adacpp native stream_ifc_to_step produced no solids for {ifc_path}: {stats}")
-    if stats.get("faces_dropped", 0) or stats.get("drop_reasons"):
-        logger.warning("native IFC->STEP dropped geometry: %s", stats.get("drop_reasons"))
+    # NO GEOMETRY LEFT BEHIND: the analytic reader only represents B-rep/analytic ng:: geometry. If any
+    # product carried geometry it couldn't read (IfcExtrudedAreaSolid/CSG/tessellated -> products_skipped),
+    # OR a face was dropped, the native output is INCOMPLETE — raise so the caller falls back to OCC.
+    skipped = stats.get("products_skipped", 0)
+    if skipped or stats.get("faces_dropped", 0) or stats.get("drop_reasons"):
+        raise RuntimeError(
+            f"native IFC->STEP incomplete for {ifc_path} (products_skipped={skipped}, "
+            f"faces_dropped={stats.get('faces_dropped', 0)}, reasons={stats.get('drop_reasons')})"
+        )
     logger.info("adacpp-native IFC->STEP: solids %s/%s -> %s", stats.get("solids_out"), stats.get("solids_in"), out_path)
     return stats

src/ada/comms/rest/converter.py

@@ -1770,16 +1770,22 @@ def _via_ifc_to_step(
 
     A native IFC advanced-B-rep reader (analytic surfaces/curves + IfcMappedItem instancing) builds
     ng:: neutral geometry which the AP242 STEP emitter re-writes (instances baked). The declared length
-    unit is preserved. Raises (so the dispatcher can surface it) if the native verb is absent — the
-    generic OCC ifc→step path is the registry default in that case.
+    unit is preserved. Falls back to the OCC path (``_via_ada_to_step``) when the native reader can't
+    fully cover the file — IFC with IfcExtrudedAreaSolid / CSG / tessellated geometry, or any
+    product/face left behind — so no geometry is silently lost.
     """
-    from ada.cadit.step.native_ifc_to_step import native_ifc_to_step
+    from ada.cadit.step.native_ifc_to_step import native_ifc_to_step, native_ifc_to_step_available
     from ada.config import logger
 
-    out_path = pathlib.Path(tempfile.mkstemp(suffix=".step")[1])
-    stats = native_ifc_to_step(src_path, out_path, on_progress=on_progress)
-    logger.info("native IFC->STEP: %s", stats)
-    return out_path
+    if native_ifc_to_step_available():
+        try:
+            out_path = pathlib.Path(tempfile.mkstemp(suffix=".step")[1])
+            stats = native_ifc_to_step(src_path, out_path, on_progress=on_progress)
+            logger.info("native IFC->STEP: %s", stats)
+            return out_path
+        except Exception as exc:  # noqa: BLE001 - native can't cover this IFC; use the OCC writer
+            logger.info("native IFC->STEP unavailable/incomplete (%s); using OCC", exc)
+    return _via_ada_to_step(src_path, ".ifc", on_progress)
 
 
 def _via_step_stream_to_ifc(