Changelog

Breaking changes

• Share now defaults to ShareConfig.ResetOnCompletionAndRefCountZero, matching Rx's Publish().RefCount(): the source is disconnected when the last subscriber leaves, and a completed connection is restarted on the next subscription. Pass new ShareConfig() to keep the previous behavior.
• Merge overloads gained an optional MergeOptions parameter. This is source compatible, but code compiled against 1.x must be recompiled.

Performance

• AsyncGate uses a pooled FIFO queue of waiters instead of SemaphoreSlim plus a continuation. Each wait allocates much less, and a release hands the gate straight to the first waiter, so no other caller can jump the queue.
• A single AsyncLocal for the whole library (ExecutionFlow) replaces the per-observer, per-gate and per-subscription ones, so the ExecutionContext stays small in long pipelines.
• Linked cancellation tokens are reused across notifications instead of creating a CancellationTokenSource for every value.
• Consecutive synchronous Select/Where calls are fused into a single operator.
• On net6+, OnNextAsync and the Merge forwarding methods use PoolingAsyncValueTaskMethodBuilder.
• New MergeOptions.DetachInnerCancellation option avoids linking a token per value in Merge.

Fixes

• Share: an observer that joins or leaves during a subscription no longer resets the connection. Subscribe and connect failures are handled more reliably.
• Debounce / Throttle: OnErrorResume is no longer forwarded after termination.
• TakeUntil, Debounce, Throttle, Timeout: callers waiting on the gate are released when the subscription is disposed (in Timeout, only for OnErrorResume).
• TakeUntil: errors are reported to UnhandledExceptionHandler instead of being silently swallowed.
• Merge(IEnumerable): subscribing now waits until all sources are subscribed, and the startup method is no longer async void.
• UnhandledExceptionHandler.Register is thread-safe and rejects null.

Infrastructure

• global.json pins the .NET 10 SDK, and the CI workflows install the SDK version it specifies.