feat: add WitnessLatestOn, run sequencers from a TimeProvider, add a shared IsImmediate check

[glennawatson]

Summary

Sequencers run from a TimeProvider, hopping to a sequencer can deliver only the newest value, and one check tells you whether a sequencer is immediate.

• WitnessLatestOn(sequencer) delivers only the newest value. You get the latest value instead of a replay of every queued one.
• WitnessLatestOnSignal<T> is a public sink under Advanced. You construct it directly, like the other operator sinks.
• The thread pool, task pool, synchronization context, WebAssembly, immediate and current-thread sequencers take a TimeProvider. The time-based operators that take a sequencer run from that provider.
• VirtualClock is a TimeProvider. A test drives time from a fake provider or from a VirtualClock.
• WasmScheduler in the Reactive package takes a TimeProvider.
• sequencer.IsImmediate tells you whether a sequencer is the immediate one. It works in both the lean and the Reactive packages.

Why

A two-way binding through a queuing sequencer livelocked in ReactiveUI.Binding, because WitnessOn replays every queued value. Binding also could not drive Throttle(TimeSpan, ISequencer) from a TimeProvider fake clock, and comparing a scheduler to Sequencer.Immediate misses the Reactive package's immediate scheduler.

Breaking changes

None.

How this was verified

Tests cover every type this adds and every changed sequencer, and they drive time with VirtualClock.

Notes for the reviewer

Start with WitnessLatestOnSignal{T}.cs and the sequencer constructors. The public API baselines are generated.

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

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
99.5% 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 99.78%. Comparing base (caae6b7) to head (1fbb084).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

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  Coverage   99.78%   99.78%            
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  Files         781      785     +4     
  Lines       24558    24774   +216     
  Branches     2886     2910    +24     
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+ Hits        24504    24720   +216     
  Misses         54       54            

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