Split ephemeral Relay internals

[AustinKelsay]

Parent

#130

What to build

Separate ephemeral Relay connection, client-session, and filter-matching responsibilities into private internal modules while preserving the small existing testing entrypoint.

Acceptance criteria

• Connection, client-session, and filter-matching responsibilities have distinct internal owners.
• The testing entrypoint exports and observable ephemeral Relay behavior remain compatible.
• No new production root exports are introduced.
• Focused behavior tests cover connection lifecycle, session messaging, and filter matching.
• Build, pack, Node, web, and Bun compatibility remain green.

Blocked by

• Move behavior tests off private shapes #138

Delivery

This is cleanup slice 9 of 9. Deliver it on its own branch and non-draft pull request into staging, then merge it before starting the next slice.

[AustinKelsay]

Completed and merged into staging via PR #148 (merge commit 9c9e14c). All local gates and all four hosted CI jobs are green; Grok approved; CodeRabbit actionable findings were resolved, with the hosted review timeout documented on the PR.