Add authored Poison Wait alert-notebook template (#4223)

[erikdarlingdata]

Refs #4223.

Why

Poison Wait's alert-notebook link fell back to the mechanical read-list conversion instead of an authored, forensic layout, and its wait type (THREADPOOL, RESOURCE_SEMAPHORE, ...) lived only in prose (the detail heading / detail_text), not as data a template could branch on.

What changes

• AlertContext gains a WaitType member, persisted as a trailing/nullable field on AlertContextDto (same backward-compatible shape as Route/Routing) with a TryReadWaitType reader. Both engines' Poison Wait fire sites set it: SQL Server's AlertEngine.CheckPoisonWaitsAsync (the worst-graded finding's WaitType) and PostgreSQL's DarlingWorker.EvaluatePgPoisonWaitAsync (finding.Subject) — the same value each engine's mute context already keys on.
• New AlertNotebookEndpoint.Templates.PoisonWait.cs registers authored/poison-wait for the "Poison Wait" metric (both engines).
• One row added to s_authoredTemplates, kept sorted.
• Two tests used "High CPU" as their example of a metric that stays on the mechanical notebook; High CPU gets its own notebook in Add the authored High CPU alert-notebook template (#4223) #4420, so both now use "tempdb Space", which no Alert notebooks: authored templates for the remaining families, including custom rules and analysis findings #4223 family covers.

Cell list (RESOURCE_SEMAPHORE firing):

Cell	Type	Notes
Header, Status	header/status	as every family
Top waits (SQL Server)	read get_wait_stats
Top waits (PostgreSQL)	read get_pg_wait_stats	wrong-engine read answers "not applicable" on its own
Waiting tasks	read get_waiting_tasks
Resource semaphore	read get_resource_semaphore	ONLY when wait type == RESOURCE_SEMAPHORE (ordinal, case-insensitive)
Memory grants	read get_memory_grants	same gate
Wait time over time	panel, wait_stats/wait_time_ms (sum), filtered to the wait type when known
Wait trend	read get_wait_trend	bound to the incident's exact wait type; omitted (replaced by a markdown note) when no wait type is known, since the read requires one

Test plan

New Darling/Darling.Tests/AlertNotebookTemplatePoisonWaitTests.cs (13 facts/theories): routing, RESOURCE_SEMAPHORE gate (present + absent, case-insensitive), get_wait_trend's wait_type equals the row's exactly, the no-wait-type degrade (omits trend, adds a note, same cell count), ValidateNotebookDefinition passes both with and without a wait type, and the context-serializer round-trip (WaitType persists and rehydrates; legacy JSON with no member reads null).

RED on dev: AuthoredTemplate_PoisonWait_IsNotNull fails today because "Poison Wait" has no registered row (AuthoredTemplate returns null).

GREEN: dotnet Darling.Tests.dll -class Darling.Tests.AlertNotebookTemplatePoisonWaitTests -class Darling.Tests.AlertNotebookAuthoredTemplateTests -class Darling.Tests.AlertNotebookEndpointTests -class Darling.Tests.DocCommentHygieneTests on macOS: Total 159, Failed 0.

Both test projects build clean: Darling.Tests and Lite.Tests at 0 errors (-p:EnableWindowsTargeting=true). Lite.Tests/Darling.Tests themselves are Windows-only and were not run beyond the in-process class list above; CI decides the rest. Full suite was not run given the time budget — the class list above covers everything the family's shared theories and this PR's own file touch.

CHANGELOG entry

SECTION: Added
ENTRY:

• Poison Wait alerts open an authored notebook: top waits on both engines, waiting tasks, resource-semaphore/memory-grant detail when the wait is RESOURCE_SEMAPHORE, and a trend bound to the firing wait type. ([Add authored Poison Wait alert-notebook template (#4223) #4424])
  REF:
  [Add authored Poison Wait alert-notebook template (#4223) #4424]: Add authored Poison Wait alert-notebook template (#4223) #4424