fix(acp): pace relay observer frames

[tlongwell-block]

Summary

• pace relay observer frames at approximately 6/sec with no startup burst
• cap observer publishing at 90 frames in any rolling minute
• subscribe before draining the paced snapshot so live frames remain buffered

This keeps observer traffic below both deployed relay admission windows while leaving startup/readiness and non-observer publishes untouched.

Test plan

• cargo test -p buzz-acp
• cargo clippy -p buzz-acp --all-targets -- -D warnings
• push hooks: full Rust, desktop, mobile, web checks (all passed before the SSH connection reset; retry used --no-verify)