F-39: NVS writes on the LVGL task #42

Severity: LOW · Effort: Small · Batch: 5 · Status: Code done in #21 (29b54d2), awaiting bench verification

Found in the second full review of the firmware. Recorded with the rest of the pass (F-19…F-44) in docs/REVIEW_REMEDIATION_PLAN_2.md, which is where its status is kept.

JmriConfigScreen::saveSettings, the orchestrator screen's save and saveSpeedSteps still
write NVS from event handlers, against F-05's rule.

Progress: the JMRI settings are now saved by JmriConnectionController on its own task
(batch 3, F-34). Batch 5 moved the rest.

Acceptance Criteria

• SettingsWriter — one task, settings_writer, and a queue — carries out every NVS
  write the UI asks for, in order: speed steps, the transport choice, the orchestrator
  settings, WiFi Forget, and the JMRI settings under the orchestrator (replacing the
  one-shot jmri_save task).
• The orch read-modify-writes happen on the writer, so a transport change and an
  orchestrator Save moments apart cannot overwrite each other with stale copies (the
  transport choice's own save was a fourth NVS write on the LVGL task, not listed above).
• The orchestrator screen's Connect posts the supervisor's wake-up behind its save, so
  the login reads what was just typed.
• Bench: change speed steps and leave the screen at once; the new value applies. Save
  orchestrator settings, then Connect; the login uses them. Forget WiFi.