Serialize fan-out EVENT frames once

[tlongwell-block]

Summary

• Cache live fan-out EVENT frames as immutable Arc<Bytes> per unique subscription ID within each fan-out cycle.
• Send cached UTF-8 text bytes through ConnectionManager without changing existing string senders.
• Keep frame construction strictly after filter_fanout_by_access; the persistent DM visibility owner fence is applied before frame caching too.

W5 fences

1. Byte-identical frame: event_frame_for_sub preserves the old format!(r#"["EVENT","{}",{}]"#, sub_id, event_json) wire shape; unit test asserts String and Bytes equality against that legacy format.
2. Cycle-scoped sharing: each fan-out call builds a fresh local HashMap<&str, Arc<Bytes>>; unit test asserts duplicate sub IDs share within one cache and a new cache gets a distinct Arc.
3. No access widening: all three live fan-out paths build the cache only after filter_fanout_by_access returns; the persistent DM visibility owner gate is also applied before caching/sending.

Validation

• bin/rustup run 1.95.0 cargo fmt --check
• bin/rustup run 1.95.0 cargo test -p buzz-relay fanout_
• bin/rustup run 1.95.0 cargo test -p buzz-relay
• bin/rustup run 1.95.0 cargo clippy -p buzz-relay --all-targets -- -D warnings
• pre-push hook with repo-local Hermit path: passed rust/desktop/mobile suites

Ships on 2df85d3f70459c529ceab67ccd26b25967107478.