feat: add GroupBy, GroupByUntil and Window/Slice operators

[glennawatson]

Summary

Adds keyed grouping and windowing operators, and fixes two defects in the extensions package.

• GroupBy and GroupByUntil deliver a keyed GroupedSignal<TKey, T> per key. They cover the key, element and duration selector forms, and each group completes when its source or its duration signal completes.
• Window and Slice hand back a signal per window. They cover count, count with skip, time, time with count, boundary, opening and closing forms. Slice is the primary name and Window is the migration alias.
• Every new signal and witness is public under Advanced. Each is constructible directly, like BufferSignal<T>.
• SkipWhileNull accepts nullable reference element types. Calling it on IObservable<string?> raises no nullability warning.
• LatestOrDefault documents what it emits and when.

Why

Grouping and windowing are the largest operator families a System.Reactive user looks for and does not find. The SkipWhileNull constraint forced a suppression at call sites with nullable element types.

Breaking changes

None.

How this was verified

Tests cover every class this adds, using deterministic ordered state checks with a virtual clock for the time-based windows. The SkipWhileNull change has a test on a nullable reference source.

Notes for the reviewer

• Start with GroupedSignal{TKey,T}.cs and SliceRouter{TOuter,T}.cs, which the group and window signals share.
• The PublicAPI.txt baselines, the README rows and the per-signal witness files follow one pattern and are safe to skim.
• The async mirror of these operators and the remaining System.Reactive parity operators are out of scope for this PR.

Checklist

• I have read the Contribute guide
• The PR title follows Conventional Commits
• Tests cover this change, or the summary says why they do not
• New or changed public API has XML documentation

[codecov]

Codecov Report

❌ Patch coverage is 99.00881% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.78%. Comparing base (aacf8d8) to head (a19c0c0).

Files with missing lines	Patch %	Lines
...ByUntilWitness{TSource,TKey,TElement,TDuration}.cs	96.33%	4 Missing ⚠️
.../Primitives.Shared/Advanced/SliceTimeWitness{T}.cs	97.87%	2 Missing ⚠️
...itives.Shared/Advanced/SliceTimeCountWitness{T}.cs	98.75%	1 Missing ⚠️
...s.Core/Advanced/SliceClosingWitness{T,TClosing}.cs	98.75%	1 Missing ⚠️
...vanced/SliceOpeningWitness{T,TOpening,TClosing}.cs	98.92%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #219      +/-   ##
==========================================
- Coverage   99.80%   99.78%   -0.03%     
==========================================
  Files         757      781      +24     
  Lines       23650    24558     +908     
  Branches     2774     2886     +112     
==========================================
+ Hits        23605    24504     +899     
- Misses         45       54       +9     

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
98.3% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud