fix(sub12a): rename _seed_default_inputs → action_seed_default_inputs

Odoo 19 rejects view buttons that call private (underscore-prefixed)
methods. Renamed the public entry point. The post_init_hook callers
follow.

Caught by entech upgrade (ParseError on the form view).

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

fusion_plating/fusion_plating/models/fp_step_template.py

@@ -198,9 +198,13 @@ class FpStepTemplate(models.Model):
         'gating': [],
     }
 
-    def _seed_default_inputs(self):
+    def action_seed_default_inputs(self):
         """Seed input_template_ids based on default_kind. Idempotent —
-        only adds inputs whose names don't already exist on this template."""
+        only adds inputs whose names don't already exist on this template.
+
+        Public method (Odoo 19 requires non-underscore-prefixed names
+        for methods called from a view button).
+        """
         Input = self.env['fp.step.template.input']
         for tpl in self:
             if not tpl.default_kind: