[glennawatson]

What kind of change does this PR introduce?

Feature, plus a refactor of an existing operator onto the new sink.

What is the new behavior?

SwitchMap projects each value to an inner observable and mirrors the latest one, in a single fused sink.

• SwitchMapSignal<TSource, TResult> is public in Advanced. One object and one observer hop, where a projection followed by a separate switch costs two of each plus an intermediate sequence of observables.
• Every source value switches, null included. A caller that projects null onto its own inner observable switches onto it, which detaches the previous inner and drops its subscription.
• SwitchSelect is reimplemented on the same sink. It keeps its existing behavior of skipping nulls, which leaves the active inner subscription in place. The two now differ only in that flag; the switching bookkeeping - generation ids, the gate, the outer/inner completion interlock - exists once instead of twice.
• Both flavours get it. The sink is shared source, so it lands in ReactiveUI.Primitives.Advanced and ReactiveUI.Primitives.Reactive.Advanced.

What is the current behavior?

Reaching the latest inner observable from a projection means composing two sinks.

• Select(selector).Switch() allocates the projection sink, its observer, the switch sink and its observer, and threads an IObservable<IObservable<T>> between them.
• SwitchSelect is the only fused form and it always skips nulls, so it cannot express a projection that switches on null.

What might this PR break?

None.

• SwitchMap is new API.
• SwitchSelect keeps its signature and its skip-null semantics; only its implementation moved.

Checklist

• I have read the Contribute guide
• Tests have been added or updated (for bug fixes / features)
• Docs have been added or updated (for bug fixes / features)
• Changes target the main branch
• PR title follows Conventional Commits

Additional information

Most of the diff is the public API baselines; two files were authored.

• src/Primitives.Shared/Advanced/SwitchMapSignal{TSource,TResult}.cs - the sink.
• src/Primitives.Shared/SignalOperatorMixins.ChooseSwitchSelect.cs - the SwitchMap operator, and SwitchSelect reduced to a call into the sink.
• src/tests/ReactiveUI.Primitives.Tests/SwitchMapTests.cs - covers switching on null, stale-inner suppression, the completion interlock, selector/outer/inner errors, and disposal.
• The remaining 36 files are one added block each in PublicAPI.Shipped.txt, inserted at its sorted position.

The existing SwitchSelect tests are unchanged and still pass, which is what pins the refactor.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.14%. Comparing base (8dbb7e6) to head (d98172f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #163      +/-   ##
==========================================
- Coverage   98.14%   98.14%   -0.01%     
==========================================
  Files         679      680       +1     
  Lines       21417    21432      +15     
  Branches     2619     2619              
==========================================
+ Hits        21020    21034      +14     
  Misses        197      197              
- Partials      200      201       +1     

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[github-actions]

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