refactor(jobs): address code review feedback on fp.job.step (Task 1.5)

- I2: Add TODO comment block + stub button_pause/button_skip/
  button_cancel that raise NotImplementedError. Makes the missing
  state-machine paths explicit instead of invisible gaps. Future
  Task 1.6 wires the real implementations; shop-floor buttons in
  Task 1.8 can already point to the right method names.
- I3: button_finish now preserves first-finish audit timestamp
  via 'if not step.date_finished:' guard, mirroring button_start.
  Future rework flow that re-opens a step won't lose the original
  finish data. The duration_actual rollup landing in Task 1.7 will
  use timelog rows for the most-recent interval if needed.
- I4: step_count and step_done_count are now store=True so list
  views and stat buttons (Task 1.8) don't recompute across all
  job rows on every render. step_progress_pct and current_step_id
  stay non-stored - they're cheap derivatives. Split compute methods
  so stored + non-stored fields don't share one method (Odoo flags
  the mix as inconsistent and recomputes stored fields whenever the
  non-stored one is read, defeating the perf gain).

Manifest 19.0.8.4.0 -> 19.0.8.4.1.

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': '19.0.8.4.0',
+    'version': '19.0.8.4.1',
     'category': 'Manufacturing/Plating',
     'summary': 'Core plating / metal finishing ERP: facilities, processes, tanks, baths, jobs, operators.',
     'description': """