fix: rename ambiguous 'Work Center' / 'Work Centres' menus

Two distinct entities were both labelled 'Work Centre' (US/UK spelling
the only differentiator — confusing). Renamed by purpose, model IDs
unchanged so all 12+9 existing cross-refs keep working:

fusion.plating.work.center → 'Production Line'
  Physical shop-layout grouping that owns tanks. Examples: 'Line 1 —
  EN', 'Anodize Line', 'Prep Bay'. Has tank_ids (O2M),
  supported_process_ids (M2M), capacity_per_day. The thing tanks live
  in.

fp.work.centre → 'Routing Station'
  Per-job-step routing entity (post-Sub-11 mrp.workcenter replacement).
  Has 'kind' selection (wet_line / bake / mask / rack / inspect),
  cost_per_hour for fp.job.step rollup, default_bath_id +
  default_tank_id for release-ready validation. The thing a job step
  routes through.

Conceptually a Production Line CONTAINS many Routing Stations (e.g.
'EN Line' production line has wet-line, bake, inspect routing
stations on it).

Updated:
- _description on both models
- string= on the name fields
- list/form/search view strings
- act_window names ('Production Lines' / 'Routing Stations')
- menu items in fp_menu.xml + fp_jobs_menu.xml
- doc comments in both model files explaining the distinction

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': '19.0.11.2.0',
+    'version': '19.0.11.3.0',
     'category': 'Manufacturing/Plating',
     'summary': 'Core plating / metal finishing ERP: facilities, processes, tanks, baths, jobs, operators.',
     'description': """