feat(sub12b): extend fusion.plating.rack — racking_state + tags + capacity

The existing 'state' field tracks wear (active/needs_strip/stripping/
retired). Sub 12b adds an orthogonal 'racking_state' (empty/loading/
loaded/in_use/awaiting_unrack/out_of_service) for the load lifecycle
— a rack can be wear-active AND racking-loaded simultaneously.

New fields:
  racking_state        — operational lifecycle
  tag_ids (M2M)        — fp.rack.tag chips on plant overview
  capacity_count       — soft warn (distinct from existing 'capacity')
  current_job_step_id  — compute, derived from latest fp.job.step.move
  current_tank_id      — compute
  current_part_count   — compute

Form view picks up the new fields under Sub-12b group + a 'Current
Use' panel that hides when racking_state is empty / out_of_service.

The compute references fp.job.step.move which lands in Task 4 — the
module won't load cleanly on entech until Tasks 4-5 ship; that's
expected for batch deployment at the end.

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

fusion_plating/fusion_plating/models/fp_rack.py

@@ -115,3 +115,67 @@ class FpRack(models.Model):
         """Add `delta` to the rack's MTO count. Called by the WO finish hook."""
         for rec in self:
             rec.mto_count = (rec.mto_count or 0.0) + delta
+
+    # ===== Sub 12b — racking lifecycle (orthogonal to wear-tracking state) =
+    racking_state = fields.Selection(
+        [
+            ('empty', 'Empty'),
+            ('loading', 'Loading'),
+            ('loaded', 'Loaded'),
+            ('in_use', 'In Use'),
+            ('awaiting_unrack', 'Awaiting Unrack'),
+            ('out_of_service', 'Out of Service'),
+        ],
+        string='Racking State', default='empty', tracking=True,
+        help='Operational state in the rack→step→tank flow. Distinct '
+             'from the wear-tracking `state` (active/needs_strip/...).',
+    )
+    tag_ids = fields.Many2many(
+        'fp.rack.tag',
+        'fp_rack_tag_rel', 'rack_id', 'tag_id',
+        string='Tags',
+    )
+    capacity_count = fields.Integer(
+        string='Capacity (parts) — soft warn',
+        help='Soft warning threshold — runtime informs operator when '
+             'rack is loaded beyond this. Not enforced. Distinct from '
+             '`capacity` field (planning capacity).',
+    )
+
+    current_job_step_id = fields.Many2one(
+        'fp.job.step', string='Current Step',
+        compute='_compute_current_use', store=True,
+    )
+    current_tank_id = fields.Many2one(
+        'fusion.plating.tank', string='Current Tank',
+        compute='_compute_current_use', store=True,
+    )
+    current_part_count = fields.Integer(
+        string='Parts on Rack',
+        compute='_compute_current_use', store=True,
+    )
+
+    @api.depends('racking_state')
+    def _compute_current_use(self):
+        # Walks the most recent fp.job.step.move row per rack to derive
+        # current step + tank + part count. For racks not currently in
+        # use, all values are blank.
+        Move = self.env['fp.job.step.move']
+        for rack in self:
+            if rack.racking_state in ('empty', 'out_of_service'):
+                rack.current_job_step_id = False
+                rack.current_tank_id = False
+                rack.current_part_count = 0
+                continue
+            recent = Move.search(
+                [('rack_id', '=', rack.id)],
+                order='move_datetime desc',
+                limit=1,
+            )
+            rack.current_job_step_id = recent.to_step_id if recent else False
+            # current_tank_id pulls from the destination step's tank if set
+            rack.current_tank_id = (
+                recent.to_tank_id or
+                (recent.to_step_id.tank_id if recent and recent.to_step_id else False)
+            ) if recent else False
+            rack.current_part_count = recent.qty_moved if recent else 0

fusion_plating/fusion_plating/views/fp_rack_views.xml

@@ -68,6 +68,20 @@
                             <field name="strips_count"/>
                         </group>
                     </group>
+                    <group>
+                        <group string="Tags + Capacity (Sub 12b)">
+                            <field name="racking_state"/>
+                            <field name="tag_ids" widget="many2many_tags"
+                                   options="{'color_field': 'color'}"/>
+                            <field name="capacity_count"/>
+                        </group>
+                        <group string="Current Use"
+                               invisible="racking_state in ('empty','out_of_service')">
+                            <field name="current_job_step_id" readonly="1"/>
+                            <field name="current_tank_id" readonly="1"/>
+                            <field name="current_part_count" readonly="1"/>
+                        </group>
+                    </group>
                     <group string="Notes">
                         <field name="notes" nolabel="1" colspan="2"/>
                     </group>