Perf: eliminate hot-path allocations, modernize C# usage

[glennawatson]

What's changed for end users

This release targets the core notification pipeline — every value flowing through any async operator hits ObserverAsync.TryEnterOnSomethingCall and Concurrent.Forward*. These changes reduce per-notification allocations significantly:

• ~200 bytes saved per notification when the caller's cancellation token is None or matches the dispose token (the common case for OnCompleted and most OnNext calls). Previously a CancellationTokenSource.CreateLinkedTokenSource was allocated on every single call.
• Zero-allocation single-observer fast path for concurrent subjects — the most common case (1 subscriber) now avoids Task[], Task.WhenAll, LINQ iterator, and closure allocations entirely.
• ImmutableArray replaces ImmutableList for observer tracking in all 4 subject base classes — flat array instead of balanced binary tree, better cache locality at typical observer counts (1–5).
• Async state machine removed from ObservableAsync.SubscribeAsync — was wrapping a single await for no reason.

No public API changes. No behavior changes. Drop-in upgrade.

---

Summary

ObserverAsync — the hottest path in the library:

• Introduce LinkedTokenScope record struct to avoid CancellationTokenSource.CreateLinkedTokenSource allocation (~200 bytes) on every OnNext/OnError/OnCompleted when the caller token is None or already the dispose token
• Replace locking on AsyncLocal<int> with a dedicated gate field using System.Threading.Lock on NET9+

Concurrent subject forwarding:

• Replace LINQ .Select().AsTask() with manual for-loop over pre-sized Task[] in all three Forward*Concurrently methods, eliminating iterator, closure, and Task wrapper allocations per observer per emission
• Add single-observer fast path that returns the ValueTask directly without any Task[] or Task.WhenAll overhead

ObservableAsync.SubscribeAsync:

• Remove unnecessary async state machine — was only awaiting a single call and returning the result

CompositeDisposableAsync.Clear:

• Replace LINQ .Take(clearCount) with index-bounded for-loop to eliminate enumerator allocation

Subject base classes (BaseSubjectAsync, BaseReplayLatestSubjectAsync, BaseStatelessSubjectAsync, BaseStatelessReplayLastSubjectAsync):

• Convert ImmutableList<IObserverAsync<T>> → ImmutableArray<IObserverAsync<T>> for lower overhead and better cache locality at typical observer counts (1–5)

Operator subscription safety:

• Extract SubscriptionHelper.SubscribeAndDisposeOnFailureAsync to consolidate the try/catch/dispose/throw pattern from ~20 call sites across CombineLatest arities, TakeUntil variants, Merge, Switch, Concat into one directly-tested internal helper

Modernization:

• Seal AsyncGate, simplify dispose pattern
• File-scoped namespace for WrappedObserverAsync
• Consistent ArgumentExceptionHelper.ThrowIfNull usage in DisposableAsync and ReactiveExtensions
• NET9 Lock conditional for BufferUntilIdle gate
• Fix CS1734 XML doc warning in GroupBy

Test plan

• dotnet build ReactiveUI.Extensions.slnx -c Release — 0 errors, only pre-existing SA1201 warnings from extension<T> syntax
• dotnet test --solution ReactiveUI.Extensions.slnx -c Release — 3780 tests pass across net8.0/net9.0/net10.0 (0 failures)
• Branch coverage 95.34% → 97.68% (57 new tests covering multi-observer concurrent subjects, terminal operator failure paths, subscription helper, operator error propagation, disposal edge cases, factory validation)

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 99.27536% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.92%. Comparing base (8062f23) to head (597fe1d).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...I.Extensions/Async/Internals/SubscriptionHelper.cs	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #138      +/-   ##
==========================================
+ Coverage   98.21%   98.92%   +0.71%     
==========================================
  Files         119      120       +1     
  Lines        5161     5137      -24     
  Branches      688      675      -13     
==========================================
+ Hits         5069     5082      +13     
  Misses         15       15              
+ Partials       77       40      -37     

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[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.