feat(configurator): C4 — push coating + treatments back to part catalog defaults

Adds x_fc_default_coating_config_id and x_fc_default_treatment_ids
fields on fp.part.catalog. Wizard line gets a push_to_defaults
toggle. After action_create_order confirms the SO, any line with
push_to_defaults=True writes its coating + treatments back onto the
part catalog entry as the new defaults.

Reverse direction too: onchange on part_catalog_id in the wizard
line seeds coating + treatments from the part's defaults (if set and
the line doesn't already have them).

Part catalog form gets a new "Defaults" tab showing the stored
defaults. Smoke-tested: pushing default on order 1 populates the
catalog entry; new wizard line for that part auto-seeds the coating.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

fusion_plating/fusion_plating_configurator/models/fp_part_catalog.py

@@ -131,6 +131,21 @@ class FpPartCatalog(models.Model):
     notes = fields.Html(string='Notes')
     active = fields.Boolean(string='Active', default=True)
 
+    # ---- Direct-order defaults (Phase C — C4) ----
+    x_fc_default_coating_config_id = fields.Many2one(
+        'fp.coating.config',
+        string='Default Treatment',
+        help='Default coating applied when this part is dropped onto a '
+             'direct order line. Updated when "Save as Default" is ticked.',
+    )
+    x_fc_default_treatment_ids = fields.Many2many(
+        'fp.treatment',
+        relation='fp_part_catalog_default_treatment_rel',
+        string='Default Additional Treatments',
+        help='Default additional treatments. Seeded when "Save as Default" '
+             'is ticked on a direct order line.',
+    )
+
     # Substrate density mapping (g/cm³) for material weight calculation
     _SUBSTRATE_DENSITY = {
         'aluminium': 2.70,