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/tests/test_fp_job_state_machine.py

@@ -44,3 +44,21 @@ class TestFpJobStateMachine(TransactionCase):
         job.action_cancel()
         with self.assertRaises(UserError):
             job.action_confirm()
+
+    def test_cannot_cancel_done(self):
+        # Done jobs cannot be cancelled — covers the UserError branch in
+        # action_cancel.
+        job = self._make_job()
+        job.action_confirm()
+        # Force the state to 'done' for the test (no public action yet —
+        # done is set by step-completion logic landing in Task 1.5+).
+        job.state = 'done'
+        with self.assertRaises(UserError):
+            job.action_cancel()
+
+    def test_cannot_cancel_already_cancelled(self):
+        # Idempotent re-cancel is now an explicit error.
+        job = self._make_job()
+        job.action_cancel()
+        with self.assertRaises(UserError):
+            job.action_cancel()