feat: add WatchReaders reader-join stream to v3 RelayService

[alvgaona]

Summary

The bridge needs to know when a reader joins the relay's message stream so it can replay latched topics that the relay's bounded history evicted. The relay observes joins but had no way to announce them. This adds the announcement channel to the v3 relay service as a server-streaming rpc, named for the observed fact rather than today's most common client, since a viewer is the typical reader and a data-platform uploader is another.

Changes

• WatchReaders(WatchReadersRequest) returns (stream ReaderEvent) on rewire.v3.RelayService
• ReaderEvent carries a monotonic join count and nothing else, no reader identity. The stream opens with one event carrying the current count so a freshly connected watcher learns the baseline, and a count lower than previously seen tells a watcher the relay restarted
• Generated v3 code regenerated with the pixi run generate-proto task. Unrelated codegen churn in v1 and v2 is excluded

Related

• REW-218

Checklist

• PR title follows Conventional Commits
• pixi run sanity passes locally (check + fmt + lint + test)
• cargo clippy --all-features --all-targets -- -D warnings is clean
• cargo doc --no-deps --all-features builds without warnings
• Tests added or updated for the change — the rpc is wire schema plus generated code, exercised end to end by the consuming bridge test suite

Breaking changes

None. The rpc is additive and existing clients are unaffected. Servers that predate it answer unimplemented, which the bridge treats as feature absence.