feat(fusion_plating_jobs): group WOs by recipe step structure

Replace the old 5-tuple (recipe.id, part, spec, thickness, serial) grouping
key with a structural signature so multiple parts that share the same recipe
step tree (ENP clones) collapse onto one combined work order. Add three
helpers: _fp_recipe_signature, _fp_line_express_signature, _fp_line_group_key.
Add TransactionCase test covering merge, non-merge, and masking-split cases.

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

fusion_plating/fusion_plating_jobs/tests/__init__.py

@@ -11,3 +11,4 @@ from . import test_post_shop_states
 from . import test_recipe_cert_suppression
 from . import test_order_ship_state
 from . import test_combined_cert_creation
+from . import test_wo_recipe_grouping