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>
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