feat: add Kafka exactly-once consume-process-produce

[joshua-temple]

What this change does

Fleshes out the source Transactional capability into a working Kafka exactly-once path (one of the four roadmap items from #131), so a consume-process-produce cycle commits the produced records and the consumed offset as one atomic unit.

• Core seam (source): Transactional.Begin now fences a consumed Message together with records produced through a new backend-neutral Tx handle (Tx.Produce, ProducedRecord) — emit and ack become one atomic unit. No vendor type crosses the seam.
• source/kafka: WithTransactional(id) builds a GroupTransactSession (read-committed, no auto-commit); Begin produces through the session, marks the consumed offset on success only, and ends with TryCommit/TryAbort. A narrow transactor seam keeps the begin/produce/end choreography unit-testable with a fake session.
• source/statemachine: DriveTx runs Fire + emit-as-produce + persist + offset-commit inside one transaction via a TxSink, returning Manual so the engine does not re-settle. Drive remains the at-least-once fallback; a broker abort after a durable save is covered by state-version dedup.

Why

Exactly-once consume-transition-emit is the headline guarantee of the source/statemachine bridge. On Kafka that requires the transactional producer fencing the offset commit, which only the adapter can wire — so the core seam stays vendor-neutral and the Kafka adapter owns the choreography. The capability is additive: existing at-least-once consumers are untouched.

Relates to: follows up the source Tier-1 PR (#131).

Checklist

• Commits are signed off (git commit -s) per the DCO
• Conventional commit messages (type: subject)
• Tests added (fake-session ordering unit tests + RedPanda integration EOS round-trip asserting no duplicates across an abort); source 94.5% / kafka 89.9% / statemachine 88.7%
• Public API documented (godoc + adapters & with-state docs + changelog notes)
• mage check runs in CI across the workspace and the GOWORK=off source destinations