Add authored notebook templates for the PostgreSQL-only alert families (#4223)

[erikdarlingdata]

Refs #4223.

Why

The three PostgreSQL-only alert families — Wraparound Risk, Vacuum Horizon Blocked, and Replication Slot Retention — still opened the mechanical fallback notebook (a plain list of section reads) when an alert fired. This gives each family an authored notebook instead: a header, status, its own drill-down reads, and a 24h trend panel.

What changes

New file AlertNotebookEndpoint.Templates.Postgres.cs (Darling/PerformanceMonitor.Darling.Service/), three builders registered as one row each in s_authoredTemplates:

Family (metric)	Template id	Cells
PostgreSQL Wraparound Risk	authored/pg-wraparound	Header, Status, get_pg_wraparound_risk, get_pg_autovacuum_health (limit 20), 24h trend of pg_xids_remaining (min), database-filtered
PostgreSQL Vacuum Horizon Blocked	authored/pg-xmin-horizon	Header, Status, get_pg_xmin_horizon, get_pg_session_states (limit 25), 24h trend of pg_xmin_age (max), no database filter — pg_xmin_horizon's only dimension is source
PostgreSQL Replication Slot Retention	authored/pg-replication-slot	Header, Status, get_pg_replication_slots, get_pg_replication_stats (limit 25), 24h trend of pg_slot_retained_wal_bytes (max), database-filtered

Each family's second read is the mechanical map's own second section for that metric (DarlingTriageEndpoint.cs), carried forward as an authored read cell.

The three primary reads (get_pg_wraparound_risk, get_pg_xmin_horizon, get_pg_replication_slots) declare no limit param in their catalog entry, so they're added to s_authoredLimitlessTrendReads alongside get_deadlock_trend. That set was private; widened to internal so the shared budget theory (AlertNotebookAuthoredTemplateTests.Budget_ReadCellsHaveLimitsExceptTheTrendRead...) checks membership in the real exemption set instead of hardcoding get_deadlock_trend by name — that hardcode would have false-failed on this PR's reads, so the shared test now generalizes correctly for any future exempt read too.

A new PgGaugeTimelinePanel helper (distinct from the shared TimelinePanel) builds the 24h gauge trend: no annotations key and a caller-supplied aggregate, since none of these three sources are per-event/count measures with a catalog annotation source — TimelinePanel hard-codes aggregate: "count" and always attaches an annotation, which doesn't fit a gauge.

Darling.Tests/AlertNotebookTemplatePostgresTests.cs (new, 9 tests): routing for all three metrics, the exact cell list/types/reads/params/trend source-measure-aggregate per family, and a no-matched-incident degrade (same cell count) for each. The shared theories in AlertNotebookAuthoredTemplateTests (data-driven off the registration table) already cover validation, dispatch, compose-catalog, budget, binding and viz for these three metrics with no edits needed there beyond the exemption-set generalization above.

Test plan

• RED on origin/dev (5899b85): all 9 new tests fail — AuthoredTemplate(metric) returns null for all three metrics (no row registered), so .Value on the null result throws.
• GREEN on this branch: AlertNotebookTemplatePostgresTests 9/9, AlertNotebookAuthoredTemplateTests 56/56, AlertNotebookEndpointTests 29/29, DocCommentHygieneTests 77/77 — all run in-process on macOS (pure, no live PG needed).
• Darling.Tests.csproj and Lite.Tests.csproj both build 0 errors (-p:EnableWindowsTargeting=true); both target net10.0-windows and can't run here — CI decides them.
• Did not run the full suite (out of time budget); ran the four template and notebook test classes plus the new file.

CHANGELOG entry

SECTION: Added
ENTRY:

• PostgreSQL Wraparound Risk, Vacuum Horizon Blocked and Replication Slot Retention alerts open an authored notebook: their own drill-down reads and a 24h trend panel, instead of the mechanical fallback section list. ([Add authored notebook templates for the PostgreSQL-only alert families (#4223) #4419])
  REF:
  [Add authored notebook templates for the PostgreSQL-only alert families (#4223) #4419]: Add authored notebook templates for the PostgreSQL-only alert families (#4223) #4419