[mrcsin]

Problem

The OPC UA live edge (#116) needs a second implementation of the live stream. The stream sat on
IDataProvider.Subscribe beside the history, extent, days and catalogue reads, so a second source would
have had to implement the whole archive surface.

What changed

• ILiveSource in SemiPlot.Core/Data: one cold Subscribe(IReadOnlyList<int>) of samples and breaks.
  Its contract in data-integration.md#realtime: the stream never completes and never faults, a null
  Value is a break, timestamps are UTC, an id the source does not serve emits nothing.
• PostgresLiveSource holds the archive poll, the port's first implementation. RealtimePoll,
  ArchiveStatements and the poll cadence are unchanged.
• IDataProvider loses Subscribe. TrendCoordinator and TrendWindow.Build take both interfaces; the
  coordinator keeps its Catch for a source that breaks the contract.
• Docs: data-integration.md, overview.md, charting.md, postgres-topology.md,
  trend-interaction.md, testing-strategy.md, docs/architecture/README.md, AGENTS.md.

Nothing the operator sees changes. The poll's log lines now carry the PostgresLiveSource source context.

How it was verified

• dotnet build SemiPlot.slnx: 0 errors. dotnet format --verify-no-changes and dotnet terse over the
  24 touched .cs files: exit 0.
• SemiPlot.Tests.Unit: 2303/2303. SemiPlot.Tests.Integration: 200/200.
• Guard probe: deleting IDataProvider.Subscribe at the base gives CS1061 at TrendCoordinator.cs:98;
  TheWindowFollowsTheContainersLiveSource catches a cast in place of the container's ILiveSource.
• Not run: the manual AppHost smoke check. The automated suites stand in for it in this unattended
  roadmap run.

Per-task commits before collapse

b764760 docs(plans): record the exec run and verify steps
5e8e59b refactor(bridge): trim restated comments
78d2117 docs(architecture): address review findings
2e29c5b fix(data): address review findings
c95354e docs(architecture): address review findings
9fac0ac fix(bridge): address review findings
8731098 docs(architecture): document the live-source port
a63a8e2 test(data): record the acceptance run
4940ea1 refactor(data): drop the provider's live edge
d8926c6 feat(chart): read the live edge through ILiveSource
0d20cfe test(data): subscribe the poll tests through ILiveSource
8d3cb89 feat(data): declare the live-source port