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/data/fp_job_sequences.xml

@@ -1,5 +1,8 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<odoo>
+<!-- noupdate="1" is REQUIRED — without it, every -u fusion_plating
+     resets number_next back to 1, which corrupts the live sequence
+     on every module update. Matches the convention in fp_sequence_data.xml. -->
+<odoo noupdate="1">
     <!-- Sequence for fp.job. Format: WH/JOB/00001 onwards.
          Migrated mrp.production records keep their WH/MO/... names. -->
     <record id="seq_fp_job" model="ir.sequence">