LITE: the latch and spinlock trend charts say where their data starts (part of #4966)

[erikdarlingdata]

Summary

Lite's Latches & Spinlocks tab draws a flat zero line when a trend's read is empty, so an empty stretch reads as "nothing happened". Each of the two trend charts now carries the same "Showing since" note Lite's grids carry. This is the Lite twin of the viewer's trend half of #5029.

What users see change

When the store's data begins inside the chosen window, a note above the latch chart and one above the spinlock chart reads "Showing since ", in the chart's own clock. When the window is covered, or is 90 minutes or less, nothing shows.

What changes

• QueryWindowRelation gains LatchStats and SpinlockStats (at the end of the enum and of each switch arm), mapped to v_latch_stats / v_spinlock_stats, read by collector runs (latch_stats, spinlock_stats).
• ServerTab.xaml: a banner above each chart (grid rows renumbered).
• ServerTab.LatchSpinlock.cs: after the charts bind, each banner refreshes through RefreshStoredWindowBannerAsync. A failed probe costs only its note.

Test plan

• New DataStartBannerLatchSpinlockTests: per chart a notice test and a quiet-start test, plus a relation-map test and a wiring pin. RED on the relations-only commit: Total 7, Failed 7. GREEN with the fix: Total 7, Failed 0.
• Full Lite.Tests on Windows: Total 8011, Failed 0.
• Full Darling.Tests on the Mac: 428 distinct failing names, comm -23 against the known local list prints nothing. 0-warning --no-incremental builds of both test projects.
• scrubcheck on the diff and the commit messages: 0 hits.

Limits

Both charts are zero-fill/empty-read charts, so both get the note; the snapshot grids are untouched.

CHANGELOG

SECTION: Fixed
ENTRY: - Lite's latch and spinlock trend charts say where their data starts ([#5086]) - when the store's data begins inside the chosen window, the charts show a "Showing since" note instead of a flat zero line.
REF: [#5086]: #5086