Web: the event grids' data-start note uses the earliest event shown (part of #4966)

[erikdarlingdata]

What users see change

• Blocked Process Reports, Long Query Completions, Memory Pressure Events and Default Trace now show a "partial window" note on the web server page when the data starts after the window starts.
• These grids list events by their own time, and a first collection stores a server's earlier event history. The note now names the earlier of the coverage start and the earliest event shown, as the desktop viewer does. History that reaches the window start shows no note.
• A capped Blocked Process Reports or Default Trace page names its oldest shown event, without asking the store.
• The Memory Pressure grid now draws the note on a data answer, not only on an empty one.
• Long Query Completions keeps the coverage-plus-event rule on a full answer. Its page is the slowest runs, so on a truncated page the oldest row shown says nothing about how far back the read reached: the note then uses the coverage start alone.

Changes

• ViewerEventDataStart moved from the desktop viewer to PerformanceMonitor.Darling.Storage (the web service does not reference the viewer), so the web and the viewer share one rule. Its code is unchanged; only its namespace and the using lines that name it moved.
• The capped-list rule is table-driven (CappedByRead); collection log and PostgreSQL config changes behave as before.
• memory_pressure_events gets its own probe source.

Test plan

• Live (scratch store): history before coverage names the earliest event; history reaching the window start gives no note, for all four reads.
• No store: capped Default Trace page names its oldest event_time with no store call; Long Query Completions with truncated:true still goes to the coverage probe.
• Page harness: Memory Pressure grid draws the note on a data answer.
• Existing WebDataStartNote* tests pass; two adjusted: an unlisted-read example that is now listed, and four source-scan rows for the new reads.
• Darling.Tests builds with 0 warnings.
• Every Darling.Tests class that names a changed file, against a live PostgreSQL store (90 classes), and every class that reads repository source (748 classes): every failure is one that fails only on macOS.
• After the first review: all six capped reads share one window anchor (CappedByRead), a test pins that set, a truncated Long Query Completions page reads the coverage start only, and a capped Blocked Process Reports page names its oldest event without asking the store.
• On Windows 11: the seven desktop viewer classes that use the moved ViewerEventDataStart, plus WebDataStartNoteEventReadsTests and WebDataStartNoteTests: 152 tests, 0 failed (49 skipped there: live cases that need a PostgreSQL store, run on the Mac).

CHANGELOG

SECTION: Fixed
ENTRY: - The web page's event grids say where their data starts ([#5197]) - Blocked Process Reports, Long Query Completions, Memory Pressure Events and Default Trace now show a partial-window note when the data starts after the window starts, naming the earlier of the collector's coverage and the earliest event shown, as the desktop viewer does. A capped page names its oldest event shown.
REF: [#5197]: #5197

Part of #4966