Fix concurrent subscriptions to share without replay

[thliu21]

Summary

• make first-subscriber detection and observer insertion atomic for share(replay: 0, scope: .whileConnected)
• add a gated concurrent regression test that verifies a cold source is connected exactly once
• register the regression test in the generated SwiftPM test entry point

Root cause and impact

The optimized no-replay sharing path released its recursive lock before inserting the observer. Concurrent first subscribers could therefore both observe an empty observer bag and both call connect() on the same connection. The second call assigned the connection's SingleAssignmentDisposable again and crashed with Fatal error: oldState.disposable != nil.

This change inserts the observer before releasing the lock, keeping the observer count and insertion in one critical section. Connecting to the source still happens after unlocking, so source subscription and user code remain outside the lock and the previous replay deadlock is not reintroduced.

Validation

• focused test2718ShareWithoutReplayConnectsOnceForConcurrentFirstSubscriptions macOS test
• complete AnomaliesTest and ObservableShareReplayScopeTests macOS suites (18 tests), including the four existing take(1) + share(replay: 1) + zip = deadlock #2653 deadlock regressions
• swift build --product RxSwift
• ./scripts/package-spm.swift
• git diff --check

Fixes #2718