[fix] Linking: register listeners once and remove by identity

[KuroGeo]

The first Linking.addEventListener('onOpen', callback) registration adds its callback twice, so opening a URL invokes it twice. Removing a listener also compares function source text, which can remove a different callback with the same implementation (such as closures created by a factory).

Initialize the listener list empty and remove callbacks by identity in both the subscription and deprecated removal paths. Three regressions cover one notification per registration, independent callbacks with identical source, and the deprecated API. All three fail before the fix.

Validation: complete DOM and Node suites pass (724 tests, 6 existing skips, 220 snapshots); repository formatting and lint checks pass. Flow is not verified locally: its pinned 0.148.0 macOS binary exits with architecture error -86 on this arm64 host. Upstream CI runs the checker on Linux.

AI assistance: OpenAI Codex implemented the fix and regression tests on behalf of @KuroGeo.

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