feat(fusion_plating): Express masking reference images → mask step + workstation viewer

Order-entry shortcut: when masking is toggled ON for an Express order line,
an amber "MASK" button appears to attach reference image(s)/PDF(s). The files
ride the existing _fp_apply_express_overrides_to_job path onto the job's
masking step, so the operator sees exactly what to mask — no recipe edit or
custom prompt needed.

- configurator: masking_attachment_ids on the wizard line + SO line;
  action_upload_masking_ref; override branch writes refs onto mask steps;
  amber multi-file MASK button (express_action_btns) shown when masking is on.
- jobs: x_fc_masking_attachment_ids on fp.job.step (per-step) + computed
  rollup on fp.job; office "Masking Refs" form page (readonly preview).
- shopfloor: workspace step payload carries masking_refs (sudo'd attachment
  read, rule 13m); operator sees thumbnail/PDF tiles on the mask step that
  open in Odoo's full-screen FileViewer (zoom + swipe).

Verified end-to-end on entech: SO-line refs land on the mask step + job
rollup (WO-30091); payload mask_refs shape correct (is_image, /web/image).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

fusion_plating/fusion_plating_jobs/views/fp_job_form_inherit.xml

@@ -302,6 +302,17 @@
             <xpath expr="//group[@name='x_fc_notes']" position="attributes">
                 <attribute name="invisible">1</attribute>
             </xpath>
+            <xpath expr="//page[@name='costs']" position="before">
+                <page string="Masking Refs" name="masking_refs"
+                      invisible="not x_fc_masking_attachment_ids">
+                    <div class="text-muted mb-2">
+                        Masking reference image(s)/PDF(s) attached at order entry (Express).
+                        The operator sees these on the masking step in the workstation.
+                    </div>
+                    <field name="x_fc_masking_attachment_ids" widget="many2many_binary"
+                           readonly="1" nolabel="1"/>
+                </page>
+            </xpath>
             <xpath expr="//page[@name='costs']" position="before">
                 <page string="Notes" name="notes">
                     <group>