Add DWG/DXF-native constraint persistence alongside the XRecord format

Implements docs/dwg_constraint_compatibility_design.md's staged plan in
full: materializes the sketch-constraint set as acadrust's real
AssocNetwork/Assoc2dConstraintGroup object graph on save, additive to
the existing OCS_SKETCH_CONSTRAINTS XRecord, so files also carry
constraints AutoCAD/BricsCAD recognize as their own native objects
(per Hakan's request on the upstream project). Covers every
ConstraintKind, including AssocVariable wiring for named-parameter
driving values, and clears+rebuilds on each save.

DWG round-trips the full dependency chain; DXF only keeps the
constraint-group shell since acadrust's own DXF writer deliberately
omits AssocDependency-family objects. Both behaviors are covered by
tests and documented in the design doc's progress section, along with
two other implementation-time findings (skipping the NOD-anchored
top-level network, and the DWG writer's root-node/connections wire
format).

Co-Authored-By: Claude Sonnet 5 <[email protected]>

src/scene/mod.rs

@@ -31,6 +31,7 @@ mod camera_ops;
 pub(crate) mod centerline;
 pub(crate) mod dimension_assoc;
 pub(crate) mod centermark;
+mod dwg_native_constraints;
 mod entity;
 mod group_layer;
 mod layout;