Make subject termination permanent and fix zero-capacity replay

[twittemb]

Description

Repeated termination can give existing and late subject subscribers different outcomes. Values sent after termination can also change a current-value subject or repopulate cleared replay history, and replay capacity zero currently retains one historical value.

Make the first finish or failure permanent across all six subjects and ignore later values, current-value assignments, and termination signals. Keep the last accepted current value and cleared replay history. Both replay variants now retain no history at capacity zero while delivering every live value to existing subscribers.

Preserve atomic registration and FIFO delivery outside the subject lock. Concurrent sends can still return while another sender drains their delivery; channel delivery does not wait for application code to process the value. Document these contracts and map each subject guarantee to its regression coverage.

Related: the guarantee audit in issue #61.

The commits separate permanent termination from zero-capacity replay. No public signatures, dependency requirements, deployment targets, channel implementations, or delivery-queue mechanisms change.

Validation

Validated on Apple Swift 6.4 / macOS. The committed content matches the tested snapshot exactly.

• Before the fixes, the added tests reproduced terminal-outcome disagreement, post-termination state changes and payload retention, and zero-capacity historical replay.
• Added 12 tests, including 1,200 concurrent termination races across the three throwing subjects. Expanded paused-delivery regressions across all six subjects to cover sends after termination without losing earlier accepted values.
• swift build -Xswiftc -suppress-warnings: passed.
• swift test --filter 'Async.*Subject.*Tests' -Xswiftc -suppress-warnings: 81 tests passed, including registration, cancellation, ordering, queued delivery, termination, and lifetime coverage.
• swift test --enable-code-coverage -Xswiftc -suppress-warnings: all 199 tests passed, zero failures.
• git diff origin/main..HEAD --check: passed.

A Swift 5.8 toolchain was unavailable locally; minimum-toolchain validation was not run.

Checklist

• this PR is based on the main branch and is up-to-date
• the commits inside this PR have explicit commit messages
• unit tests cover the new feature or the bug fix
• the feature is documented in the README.md if it makes sense
• the CHANGELOG is up-to-date