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/models/fp_job.py

@@ -89,6 +89,14 @@ class FpJob(models.Model):
                 vals['name'] = self.env['ir.sequence'].next_by_code('fp.job') or _('New')
         return super().create(vals_list)
 
+    # ------------------------------------------------------------------
+    # State machine — actions
+    # ------------------------------------------------------------------
+    # TODO(fp.job state-machine completeness): action_hold, action_resume,
+    # action_revert_to_confirmed (rework path) — to be added when shopfloor
+    # / rework workflows are wired up. For now, draft → confirmed and the
+    # cancel paths are the only enforced transitions; everything else is
+    # an explicit `state` write by privileged code.
     def action_confirm(self):
         for job in self:
             if job.state != 'draft':
@@ -102,7 +110,11 @@ class FpJob(models.Model):
         for job in self:
             if job.state == 'done':
                 raise UserError(_(
-                    "Job %s is done - cannot cancel."
+                    "Job %s is done — cannot cancel."
+                ) % job.name)
+            if job.state == 'cancelled':
+                raise UserError(_(
+                    "Job %s is already cancelled."
                 ) % job.name)
             job.state = 'cancelled'
         return True