Security audit finding Codex #14 ("cross-author event-ID collisions can
suppress Call-to-Play entries"). The Rust side guarantees that an event
nonce is unique within one authenticated author's history and preserves
`author_id` on every projected event, but the frontend reducer
deduplicated the merged view with `new Map(events.map(e => [e.id, e]))`
and tracked chat messages by `event.id` alone. A peer could therefore
publish, say, a Respond event reusing the nonce of another user's
Create event and make that call vanish from every viewer, or shadow
other users' chat messages.
`eventKeyOf` now builds `author_id + NUL + id` and is used for view
deduplication, event ordering ties, message deduplication and the
message id that CtpChat uses as its React key. Nomination ids
(`call_id`) were already creator scoped and are unchanged.
Test plan: `just frontend-test`. The new test feeds a Create from
Alice and a Respond from Bob sharing one nonce and expects both to
apply, then two same-nonce messages from different authors and expects
two distinct messages.
Claude-Session: https://claude.ai/code/session_017C3Nbgwpdm3YNwZhhFLHwg
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