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/models/sale_order.py

@@ -395,6 +395,66 @@ class SaleOrder(models.Model):
             return part.recipe_id
         return Node
 
+    def _fp_recipe_signature(self, recipe):
+        """Hashable structural signature of a recipe's step tree.
+
+        Two recipes with the same signature have identical processing
+        steps and can share one work order. Excludes the recipe ROOT
+        (its name carries the per-part ' — <part#>' suffix) and all
+        numeric targets — those are per-part attestation data on the
+        cert, not a batch splitter. Returns None for a missing recipe.
+        """
+        if not recipe:
+            return None
+        Node = self.env['fusion.plating.process.node']
+        kids = Node.search(
+            [('id', 'child_of', recipe.id),
+             ('node_type', 'in', ('sub_process', 'operation', 'step'))],
+            order='parent_path, sequence')
+        return tuple(
+            (k.node_type,
+             (k.kind_id.code if k.kind_id else '') or '',
+             (k.name or '').strip().lower())
+            for k in kids)
+
+    def _fp_line_express_signature(self, line):
+        """Per-line Express toggles that change which steps exist:
+        masking on/off and bake present/absent. Lines differing here
+        must not merge (the shared WO would silently drop one part's
+        masking or bake step). Free-text bake instructions are NOT in
+        the signature — both-present lines merge and the bake step
+        carries the last applied line's text (known Phase-1 limit).
+        When the Express fields are absent on a line's module, masking
+        defaults to True and bake to False, so a non-Express line groups
+        as masking-on / no-bake.
+        """
+        F = line._fields
+        masking = bool(line.x_fc_masking_enabled) if 'x_fc_masking_enabled' in F else True
+        has_bake = bool((line.x_fc_bake_instructions or '').strip()) \
+            if 'x_fc_bake_instructions' in F else False
+        return (masking, has_bake)
+
+    def _fp_line_group_key(self, line, sig_cache=None):
+        """WO grouping key. Lines with the same key ride one work order.
+
+        `sig_cache` (optional) memoises recipe-id -> signature so a
+        multi-line SO doesn't re-search the same recipe tree per line.
+        """
+        recipe = self._fp_resolve_recipe_for_line(line)
+        if not recipe:
+            return ('no_recipe', line.id)        # never merges
+        if sig_cache is None:
+            sig = self._fp_recipe_signature(recipe)
+        else:
+            if recipe.id not in sig_cache:
+                sig_cache[recipe.id] = self._fp_recipe_signature(recipe)
+            sig = sig_cache[recipe.id]
+        if not sig:
+            # A recipe with no step nodes has no structure to share —
+            # don't let empty-tree shells silently merge into one WO.
+            return ('no_recipe', line.id)
+        return ('recipe', sig, self._fp_line_express_signature(line))
+
     def _fp_auto_create_job(self):
         """Create fp.job(s) from the SO's plating lines.
 
@@ -436,37 +496,14 @@ class SaleOrder(models.Model):
             _logger.info('SO %s: no plating lines, skipping job creation.', self.name)
             return
 
-        # Group by (recipe, part, spec, thickness, serial). Lines that
-        # share ALL FIVE collapse into one WO. Bundling lines with
-        # different specs / thicknesses / serials under one WO would
-        # carry the first line's values onto the cert + sticker —
-        # silent mis-attestation. No-recipe lines still get their own
-        # group each.
+        # Group by recipe structural signature (+ per-line masking/bake
+        # toggles). Lines whose recipes have identical steps collapse onto
+        # one WO; no-recipe lines stay separate. See spec
+        # 2026-06-03-wo-grouping-by-recipe-combined-cert-design.md.
         groups = {}
-        unrecipe_idx = 0
+        _sig_cache = {}
         for line in plating_lines:
-            recipe = self._fp_resolve_recipe_for_line(line)
-            part_id = (
-                'x_fc_part_catalog_id' in line._fields
-                and line.x_fc_part_catalog_id.id
-            ) or False
-            spec_id = (
-                'x_fc_customer_spec_id' in line._fields
-                and line.x_fc_customer_spec_id.id
-            ) or False
-            thickness_key = (
-                'x_fc_thickness_range' in line._fields
-                and (line.x_fc_thickness_range or '').strip()
-            ) or False
-            serial_id = (
-                'x_fc_serial_id' in line._fields
-                and line.x_fc_serial_id.id
-            ) or False
-            if recipe:
-                key = (recipe.id, part_id, spec_id, thickness_key, serial_id)
-            else:
-                unrecipe_idx += 1
-                key = ('no_recipe', unrecipe_idx)
+            key = self._fp_line_group_key(line, sig_cache=_sig_cache)
             groups[key] = groups.get(key, self.env['sale.order.line']) | line
 
         # Order groups by min line sequence so dash-suffixes mirror SO