Bonsai: migrate viewport decorators onto canonical base

Migrate 17 legacy viewport decorators (ClashDecorator, SolarDecorator,
MeasureDecorator, ItemDecorator, GeoreferenceDecorator, NestDecorator,
NestModeDecorator, GridDecorator, LoadsDecorator, AggregateDecorator,
AggregateModeDecorator, PolylineDecorator, ProductDecorator,
WallAxisDecorator, SlabDirectionDecorator, FaceAreaDecorator,
BoundingBoxDecorator) from hand-rolled install/uninstall lifecycles
onto the canonical tool.Blender.ViewportDecorator base. The legacy
uninstall removed each handler from Blender but never cleared
cls.handlers, growing a stale-reference list across enable/disable
cycles. The base's uninstall clears the list correctly.

State-derived install methods (ItemDecorator, ProductDecorator,
LoadsDecorator, PolylineDecorator) keep an install override per the
base's documented contract.

Drop the now-redundant per-class draw_batch copies and the module-
or method-scope transparent_color defs in favour of the base helpers
introduced in the preceding commit. system/decorator.py and
boundary/decorator.py keep their installed-flag lifecycle (different
pattern, no leak) but consume tool.Blender.transparent_color.

Add an AST forward-compat guard pinning the contract structurally:
any class declaring handlers = [] (Assign or AnnAssign) must subclass
tool.Blender.ViewportDecorator. Add a runtime regression on
ClashDecorator's install/uninstall cycle.

Generated with the assistance of an AI coding tool.

src/bonsai/pytest.ini

@@ -7,9 +7,11 @@ markers =
     boundary
     brick
     bsdd
+    clash
     classification
     clip_box
     context
+    contract_guard
     cost
     covering
     debug