[mrcsin]

Problem

A break that opened at the live edge drew as a held line until the next history read covered the span.
RealtimePoll counted a q = 32 row for its debug line only and emitted it as an ordinary sample.

What changed

• Sample.Value and PenRealtimeValues.Values are nullable. A null value is a break for that pen at that
  moment, and TrendCoordinator keeps it in the batch.
• ChartRealtimeApplier appends a NaN column for a null at every layer, ahead of the coarse fold.
• RealtimePoll turns a q = 32 row into its value followed by a break 1 ms later. 1 ms is the smallest
  step that lands on a distinct OA-date axis position.
• Docs: charting.md, trend-interaction.md, data-integration.md, testing-strategy.md,
  trend-feature-spec.md, bench.md. The backlog item "A break that opens at the live edge draws as a
  held line" is removed.

This is the first slice of opc-live-edge (#116). Later slices write breaks on OPC UA session loss, on a
Bad status and ahead of a pen's first live sample through the same null sample. #116 is closed by the
opc-live-source slice, not by this one.

How it was verified

• dotnet build SemiPlot.slnx: 0 errors.
• dotnet format SemiPlot.slnx --verify-no-changes: exit 0.
• dotnet test SemiPlot.slnx: Unit 2285/2285, Integration 195/195, 0 skipped.
• With master's RealtimePoll.cs restored on the branch, 4 of 11 poll and journey tests fail.
• No manual run on the demo stand: the automated suites stand in for the manual test in this unattended
  roadmap run.

Per-task commits before collapse

700efe0 docs(plans): record the live-edge-break run
255d01d refactor(realtime): address comment audit
247b0da test(realtime): address review findings
4b305b3 fix(data): address review findings
74cc303 docs(architecture): reflow the bad-quality note
07a9431 docs(architecture): fix the bad-quality as-built note
9265840 docs(architecture): document the live edge break
1684a79 test(chart): tolerate the history anchor in the journey
43c5868 feat(data): emit a live break after a q = 32 row
a95ba7d test(chart): pin the break column through the chart
c045114 feat(chart): carry a live break through the batch