feat(numbering): add fp.parent.numbered.mixin abstract model

Atomic counter via SELECT FOR UPDATE on the parent SO row. Composes
child names as PREFIX-PARENT (bare for first) or PREFIX-PARENT-NN
(zero-padded 2-digit, then unpadded past 99). Subclasses implement
three hooks: _fp_parent_sale_order, _fp_name_prefix, _fp_parent_counter_field.

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

fusion_plating/fusion_plating/__manifest__.py

@@ -5,7 +5,7 @@
 
 {
     'name': 'Fusion Plating',
-    'version': '19.0.18.15.11',
+    'version': '19.0.18.15.12',
     'category': 'Manufacturing/Plating',
     'summary': 'Core plating / metal finishing ERP: facilities, processes, tanks, baths, jobs, operators.',
     'description': """

fusion_plating/fusion_plating/models/__init__.py

@@ -3,6 +3,7 @@
 # License OPL-1 (Odoo Proprietary License v1.0)
 # Part of the Fusion Plating product family.
 
+from . import fp_parent_numbered_mixin
 from . import fp_process_category
 from . import fp_process_type
 from . import fp_facility

fusion_plating/fusion_plating/models/fp_parent_numbered_mixin.py

@@ -0,0 +1,102 @@
+# -*- coding: utf-8 -*-
+# Copyright 2026 Nexa Systems Inc.
+# License OPL-1 (Odoo Proprietary License v1.0)
+"""Abstract mixin: derive a record's name from its parent sale order.
+
+Every model that 1:1 links to an SO inherits this mixin. The mixin
+owns the atomic counter logic so race conditions and counter drift
+are impossible. Subclasses implement three small hooks and call
+``self._fp_assign_parent_name()`` from their ``create()`` override.
+
+See docs/superpowers/specs/2026-05-12-parent-number-hierarchy-design.md
+for the design rationale.
+"""
+from odoo import fields, models
+from odoo.exceptions import UserError
+from odoo.tools.translate import _
+
+
+class FpParentNumberedMixin(models.AbstractModel):
+    _name = 'fp.parent.numbered.mixin'
+    _description = 'Fusion Plating - Parent-Number-Derived Naming'
+
+    x_fc_doc_index = fields.Integer(
+        string='Parent Doc Index',
+        readonly=True,
+        copy=False,
+        index=True,
+        help='1-based position within this parent SO. 1 = the first '
+             'child of this type for the SO; subsequent siblings get 2, '
+             '3, etc. The first sibling renders its name bare; later '
+             'siblings get a zero-padded "-NN" suffix.',
+    )
+
+    # ------------------------------------------------------------------
+    # Hooks subclasses must override
+    # ------------------------------------------------------------------
+    def _fp_parent_sale_order(self):
+        """Return the linked sale.order recordset (or empty)."""
+        return self.env['sale.order']
+
+    def _fp_name_prefix(self):
+        """Return the model's prefix (e.g. 'WO', 'IN', 'CoC')."""
+        raise NotImplementedError(
+            'Subclass must define _fp_name_prefix()'
+        )
+
+    def _fp_parent_counter_field(self):
+        """Return the counter field on sale.order for THIS model."""
+        raise NotImplementedError(
+            'Subclass must define _fp_parent_counter_field()'
+        )
+
+    # ------------------------------------------------------------------
+    # Core: atomic counter + name composition
+    # ------------------------------------------------------------------
+    def _fp_compose_name(self, parent_number, index):
+        """Pure helper: compose the name string per the design's rules."""
+        prefix = self._fp_name_prefix()
+        if index <= 1:
+            return f'{prefix}-{parent_number}'
+        if index <= 99:
+            return f'{prefix}-{parent_number}-{index:02d}'
+        return f'{prefix}-{parent_number}-{index}'
+
+    def _fp_assign_parent_name(self):
+        """Lock the parent SO, bump the counter, set name + doc index.
+
+        Returns True if assignment succeeded; False if no parent SO is
+        linked (caller falls back to the model's own legacy sequence).
+        """
+        self.ensure_one()
+        so = self._fp_parent_sale_order()
+        if not so or not so.x_fc_parent_number:
+            return False
+        counter_field = self._fp_parent_counter_field()
+        # SELECT FOR UPDATE - locks the SO row until commit, so a
+        # concurrent create on the same SO blocks here and reads the
+        # updated counter after we release. No race, no drift.
+        self.env.cr.execute(
+            f'SELECT {counter_field} FROM sale_order WHERE id = %s FOR UPDATE',
+            (so.id,),
+        )
+        row = self.env.cr.fetchone()
+        current = (row and row[0]) or 0
+        new_index = current + 1
+        self.env.cr.execute(
+            f'UPDATE sale_order SET {counter_field} = %s WHERE id = %s',
+            (new_index, so.id),
+        )
+        so.invalidate_recordset([counter_field])
+        new_name = self._fp_compose_name(so.x_fc_parent_number, new_index)
+        # Raw SQL update bypasses the immutability write() guard added
+        # in Task 11 (since this IS the legitimate assignment path).
+        self.env.cr.execute(
+            f'UPDATE {self._table} SET name = %s, x_fc_doc_index = %s WHERE id = %s',
+            (new_name, new_index, self.id),
+        )
+        self.invalidate_recordset(['name', 'x_fc_doc_index'])
+        so.message_post(body=_(
+            'Issued <strong>%s</strong> to %s #%s.'
+        ) % (new_name, self._name, self.id))
+        return True