feat(adminui): author MTConnect write-back on the driver form (Driver.MTConnect-004)

IWritable shipped behind write.enabled and docs/drivers/MTConnect.md told the operator to
set it, but MTConnectDriverForm — the only editor DriverFormMaps maps for this type, with
no raw-JSON escape hatch and no driver-instance API — exposed no write section, so the
documented enablement path was a hand edit of DriverInstance.DriverConfig in the config DB.

- Write-back section: switch + form text stating it is observation injection, that the
  Agent's AllowPut is an independent second gate, and that it does not by itself make a
  node writable over OPC UA (Tag.AccessLevel still composes CurrentWrite).
- Write added to the mirrored MTConnectConfigDto and to the form model, written in BOTH
  states — an omitted block would leave a previously-authored true standing, so the switch
  could never turn writing off again.
- The false 'MTConnect is read-only' header comment is gone; the doc names the new section.

No bUnit here, so the model/serialization seam is unit-tested both sides: the AdminUI test
pins the emitted bytes, and MTConnectFactoryTests parses that same document and asserts
MTConnectDriverOptions.Write.Enabled. Live page verification remains a rig step.

docs/drivers/MTConnect.md

@@ -276,7 +276,10 @@ for why Interfaces was rejected as the write-back shape. Design record:
    `AllowPutFrom = <host list>` so only named hosts may write; that list is the Agent's
    own access control and this driver neither reads nor influences it.
 2. **The driver** — `"Write": { "Enabled": true }` in the driver config (default
-   `false`; an absent block is disabled).
+   `false`; an absent block is disabled). Authored in the AdminUI on the `/raw` driver
+   config modal's **Write-back** section (`MTConnectDriverForm`), which writes the block
+   explicitly in both states — so turning the switch back off clears a previously
+   authored `true` rather than leaving it standing.
 
 Either gate alone leaves the shipped read-only behaviour unchanged: with `Write.Enabled`
 off every slot is refused `BadNotWritable` without an Agent being touched, and with