LITE: the Overview blocking chart says where its data starts (part of #4966)

[erikdarlingdata]

Summary

Lite's Overview blocking and deadlock chart draws event counts, so an empty stretch before a series starts read as "nothing happened". The chart now carries a "Showing since " note above it when its data starts after the window does. This is the Lite twin of the desktop viewer's #5024 and the last surface of #4966's Lite list.

What users see change

• A note above the Blocking & Deadlocking chart, worded like the grids' notes, in the chart's own clock (the tab's display zone, the same one the axis uses).
• One note per chart. It names the LATER of the two series' starts, so it is true of both.
• No note when the store covered the window, however late the first bar is. No note on a window of 90 minutes or less.

What changes

• LiteBlockingLaneDataStart (new, static, no WPF): each series' start is the earlier of its coverage floor and its earliest bar with a count above zero; a probe that throws costs only its series' start and is logged.
• CorrelatedTimelineLanesControl: after every chart, the ghost lines and the axis sync are drawn, the note step probes BlockedProcessReports and Deadlocks over the same UTC window the two reads took (LocalDataService.GetTimeRange). The probes are outside the reads' WhenAll; a probe failure never touches the bars.
• XAML: an Auto row holds the banner, with DynamicResource foreground.

Test plan

Lite.Tests, on the Windows VM:

• At head 257213d65: the three new classes, 16 tests (logic 7, live store 5, wiring pin 4), 0 failed.
  • OverviewBlockingLaneDataStartTests (logic): the later of the two starts; one series only; a failed probe on one side; zero buckets ignored; neither gives no note; plus the probing half of the note step, driven without WPF.
  • OverviewBlockingLaneDataStartLiveTests (store, fixed anchors): the notice, the quiet start, one series, a failed probe and a short window, through the real note step.
  • OverviewBlockingLaneDataStartPinTests (wiring): the refresh calls the note step after the blocking chart is drawn; the step probes both relations over the reads' window and passes the bar times; the XAML carries the banner.
• RED with the note step's body emptied, measured before the last three commits: the live class failed 5 of 5 and the pin class failed 1. The logic class stayed green then (5 tests, which test the choice, not the step), so the two probing-half tests were added after.
• Full Lite.Tests on the VM, measured before the last three commits: 8,027 total, 0 failed. Those commits move the banner write off the probing half, so CI's four Lite shards are the full run at the head. Builds with 0 warnings.
• Merged with dev (which now carries LITE: the Blocking Trends, Blocking Stats and Lock Wait Trend charts say where their data starts (part of #4966) #5096) before opening.

Limits

• The comparison ghost lines do not count toward the note.
• Blocking and deadlock probes ask the shared coverage relations; no new relation was added.

CHANGELOG

SECTION: Fixed
ENTRY: - The Overview blocking and deadlock chart now says where its data starts ([#5111]) - a "Showing since" note appears above the chart when the data begins after the window does, so an empty stretch no longer reads as "nothing happened".
REF: [#5111]: #5111