fix: detach a subscription's listeners when it ends

[brainRottedCoder]

Summary

Ending a DDP subscription now releases the listeners it registered. The entry carries the callbacks its subscribers passed, a re-established entry inherits that list, and forgetSubscription detaches them. A stopped stream no longer looks live.

Closes #396

Proposed changes

• ADR-0011: ending the stream ends its callbacks. The record lists them, a re-established record inherits the list, and every path that removes the record detaches them. A function shared by two records on one name stays attached until the last of them goes. A listener registered by another route, onStreamData included, is not the record's and stays.

Changes

• writeSubscription records every callback on the entry, and a reopen inherits that list instead of registering again.
• The outbound sub frame is built from id, name and params, so the callback list cannot be serialized onto the wire.
• forgetSubscription detaches the entry's callbacks. A function still listed by another entry on the same stream name stays attached.
• onEvent on a handle whose stream has already ended attaches nothing.
• Specs cover the reopen path and the teardown path separately.

Test plan

• npm run lint
• npm run typecheck
• npx jest — 483 tests, 35 suites
• CI on this pull request (lint, typecheck, test)

[CLAassistant]

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