Document how to add a setter/getter to the DataModel (review)

Answer the reviewer question "how does a developer add a new function": add a
"Adding a new setter/getter" section to the lazy layer docstring listing the
three steps (wrapper method, public method, spec entry) and noting that
test_lazy catches a missing spec entry in CI.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Ramakrishna Prabhu <ramakrishnap@nvidia.com>

Author: ramakrishnap-nv

python/cuopt/cuopt/routing/_lazy.py

@@ -21,6 +21,17 @@
     serializing a problem imports no CUDA/cuDF -- the wrapper is imported lazily
     only when a device build actually happens. ``test_lazy`` fails if this
     declared surface drifts from the wrapper.
+
+Adding a new setter/getter to the DataModel:
+
+  1. Implement it on the C++ ``data_model_view_t`` and the Cython wrapper
+     (``vehicle_routing_wrapper.pyx``).
+  2. Add the public method in ``vehicle_routing.py`` -- validation and
+     docstring -- forwarding to ``super()``.
+  3. Add its name to ``_SETTERS`` (a mutator) or ``_GETTERS`` (a query) below.
+
+``test_lazy`` cross-checks these names against the wrapper's surface, so a
+missing entry fails CI rather than silently dropping the call's data.
 """
 
 import threading