chore(acp): trace channel delivery boundaries

[jamesjlopez]

Summary

• add structured relay traces for the channel subscription, relay receipt, and harness-forwarding boundaries
• add a websocket-level regression test proving a matching channel-subscription event reaches the harness queue

Why

This diagnostic change supports investigation of #3105 without changing delivery behavior.

Verification

• cargo fmt --check
• cargo test -p buzz-acp channel_event_is_forwarded_from_matching_subscription -- --nocapture
• cargo test -p buzz-acp
• cargo clippy -p buzz-acp --all-targets --all-features -- -D warnings

[github-actions]

🔐 Codex Security Review

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