Add struct_constraints option to py-piscem mapping engine

Expose the --struct-constraints mode via the Python API:
  engine = index.mapping_engine(struct_constraints=True)

Split ConcreteMappingEngine (generic over S: SketchHitInfo) from
ConcreteAtacEngine (always SketchHitInfoSimple + binning) to avoid
type mismatches between generic caches and ATAC-specific functions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: rob-p

crates/py-piscem/piscem.pyi

@@ -105,6 +105,7 @@ class ReferenceIndex:
         strategy: str = "permissive",
         max_hit_occ: int = 256,
         max_read_occ: int = 2500,
+        struct_constraints: bool = False,
     ) -> MappingEngine: ...
     def vcolor_engine(
         self,