Release abandoned subject subscriptions and eliminate leaked test consumers

[twittemb]

The subject tests cited in #43 left consumer tasks awaiting channels after the tests ended. Those tests now finish their subjects and await their consumer tasks. Subject iterators also unregister when their last copy is released, so an early loop exit or abandoned iterator no longer leaves a subscriber channel buffering future values.

A shared registration token preserves iterator-copy semantics and existing cancellation handling across all six subject types. Regressions prove that the last iterator copy unregisters and that abandoned passthrough iterators release buffered reference values. Documents the consuming task's lifetime responsibility.

Validation on Swift 6.4/macOS:

• The iterator-lifetime regression failed for all six subjects before the fix.
• All 175 tests pass, including concurrent send ordering and cancellation/deadlock regressions.
• The cited AsyncCurrentValueSubjectTests.test_send_pushes_values_in_the_subject changed from two leaked allocations to 0 leaks / 0 bytes under leaks --atExit.
• A full-suite leak scan reports no subject/channel allocations. It still reports two 16-byte Lock.allocate() allocations in the Swift Async Algorithms 1.0.0 dependency, reached by the two- and three-input merge compatibility tests. Those dependency allocations are outside this PR's subject lifetime fix.

Fixes #43.