Add the Custom-rules alert notebook template (#4223)

[erikdarlingdata]

Refs #4223.

Why

A custom alert rule fires as Custom:<id>, and until now that metric name fell through to the mechanical notebook conversion — no chart of the rule's own measure, no threshold or band shown against the window that fired it.

What changes

Registers a Custom: prefix row (AuthoredContextKind.CustomRule) in the endpoint's authored-template table, which was empty. Adds a new file, AlertNotebookEndpoint.Templates.CustomRules.cs, with the template's cell builder.

Cell	Contents
Header	Standard header (metric, server, incident info)
Status	Standard status
Panel (normal path)	One composed panel: the SAME source/measure-or-ratio/aggregate/filters as the rule's stored plan, forced to timeBucket: "hour" (the rule's own plan is always Scalar), absolute range over the alert window, and up to 4 thresholds reference-line values carrying the rule's warn/critical (scalar op) or lower/upper bound plus optional critical band (range op)
Markdown note (deleted rule)	"This custom rule no longer exists; it may have been deleted after this alert fired." — never an error, per #2710
Markdown note (unparseable definition)	"This custom rule's definition could not be read; it may reference a measure that no longer exists." — never a throw

The panel's thresholds key is the same render-only reference-line array a composed panel already accepts; there is no separate "band" key on that schema, so a range rule's bounds ride in that same array (checked against ComposeSpec and DarlingWebEndpoints.ValidateNotebookDefinition).

Test plan

New file Darling/Darling.Tests/AlertNotebookTemplateCustomRulesTests.cs, 11 pins:

• Exact-vs-prefix routing (Custom:42 routes to authored/custom-rule; an exact-name metric still wins; Custom with no colon stays mechanical).
• A scalar rule (gt, warn 80 / critical 95): panel's source/measure/aggregate identical to the stored plan, timeBucket set, thresholds [80, 95], and it passes ValidateNotebookDefinition.
• A range rule (between, 10/90): thresholds [10, 90], and it passes ValidateNotebookDefinition.
• A deleted rule (CustomRuleMissing = true): note cell, 3 cells total (header/status/note), passes validation.
• An unparseable definition (bad JSON, and separately a definition referencing a measure that no longer exists): note cell, no throw.
• One fabricated-context theory covering the Custom: prefix family, since the shared AllAuthoredMetrics theories in AlertNotebookAuthoredTemplateTests only walk the exact-name table.

Run results (this Mac, in-process xunit v3 runner, Microsoft.WindowsDesktop.App framework entry stripped from the runtimeconfig per the repo's macOS test recipe):

• AlertNotebookTemplateCustomRulesTests: 11/11 pass.
• AlertNotebookAuthoredTemplateTests: 128/128 pass.
• AlertNotebookEndpointTests: 29/29 pass.
• DocCommentHygieneTests: 77/77 pass.
• AlertNotebookAuthoredContextTests: 26/26 on a live rig. Its ProductionPrefixTable_IsEmptyInThisStep became ProductionPrefixTable_HoldsExactlyTheRegisteredContextFamilies, which pins the single Custom: / CustomRule row this PR registers.

RED proven: copied the new test file onto pre-fix origin/dev (ef7e75df4) in a detached worktree, built clean (0/0), ran the same class: 9 of 11 fail (the two routing negatives that don't touch Custom: still pass), because s_authoredPrefixTemplates was empty there — ResolveAuthored("Custom:42") returned null.

Both Darling.Tests and Lite.Tests build 0 warnings-as-errors-relevant/0 errors (Lite.Tests is build-only here; it targets net10.0-windows and cannot run here).

CHANGELOG entry

SECTION: Added
ENTRY:

• Custom-rule alerts open a notebook that charts the rule's own measure with its threshold or band ([Add the Custom-rules alert notebook template (#4223) #4433])
  REF: [Add the Custom-rules alert notebook template (#4223) #4433]: Add the Custom-rules alert notebook template (#4223) #4433