[mrcsin]

Problem

The opc/ section added by #145 had no editor. A site moving the SCADA's OPC UA server had to edit
opc/opc.yaml by hand. The startup-failure window offered the settings dialog on an opc/ failure, but the
dialog could not edit opc/.

What changed

• SettingsSave reads and saves a third section through OpcUaConnectionLoader. The sections travel as one
  ordered IReadOnlyList<SectionRead>.
• The dialog gains an "OPC UA connection" block after the database block, with host (the IPv4 rule) and port
  (1-65535).
• The existing rules hold: only keys that already exist are edited, each in the file that owns it, and only
  the keys the operator changed. A staged copy is validated before any file is replaced. A change takes effect
  at the next start.
• The OPC fields have their own invalid messages, SettingsOpcHostInvalid and SettingsOpcPortInvalid,
  because the message line sits beside the buttons. SettingsKeyAbsent now names the section, because host
  and port exist in two sections.
• StartupFailureViewModel offers the dialog only when all three sections read and carry every key the dialog
  edits.
• Docs: AGENTS.md, overview.md, data-integration.md, ui-text.md, ui-theme.md (the measured message
  widths), charting.md, readme.md. backlog.md gets one entry: split SettingsViewModel, now 512 lines.

How it was verified

• dotnet build SemiPlot.slnx: 0 errors, only the AppHost's two NU1902. dotnet format --verify-no-changes
  and dotnet terse exit 0.
• SemiPlot.Tests.Unit: 2529/2529. SemiPlot.Tests.Integration: 205/205.
• The opc-missing-directory failure-window case failed before the gate change and passes after it.
• The dialog was measured headless with real Skia and HarfBuzz drawing. It is 640 x 575 px in both languages,
  and no message is trimmed. The tightest line is the English SettingsKeyAbsent with the OPC UA header:
  445.6 of 457 px.
• Not run: the manual smoke steps and the docs/img/settings.png recapture. The automated suites stand in
  for them in this unattended roadmap run.