refactor(jobs): address code review feedback on fp.job

- Sequence: add noupdate="1" to fp_job_sequences.xml. Without it,
  every module update resets number_next to 1, corrupting the live
  job-number stream. Matches fp_sequence_data.xml convention.
- action_cancel now raises UserError on an already-cancelled job
  instead of silently rewriting state. Audit-grade traceability
  expects explicit failures.
- Added TODO marker for action_hold / action_resume /
  action_revert_to_confirmed so future authors don't bypass the
  state-machine guards.
- Tests: added cannot_cancel_done (covers the dead-code UserError
  branch) and cannot_cancel_already_cancelled.

Manifest version bumped 19.0.8.2.0 -> 19.0.8.2.1.

Part of: native job model migration (spec 2026-04-25)
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': '19.0.8.2.0',
+    'version': '19.0.8.2.1',
     'category': 'Manufacturing/Plating',
     'summary': 'Core plating / metal finishing ERP: facilities, processes, tanks, baths, jobs, operators.',
     'description': """