feat: type-generic registry for cross-service read-your-writes subscriptions

The shared core of Shiny's cross-service read-your-writes GraphQL subscriptions
(ADR-0012), extracted and hardened from the near-identical hand-rolled handlers
in authz-service (availableCompanies) and accounting-service (entryBasesChanged)
before a third copy is written.

Registry[T] owns the keyed subscriber map, non-blocking buffered fan-out (sends
under the read lock so a close can't race a send), a key-sharded worker pool that
runs the read-view gate OFF the AMQP delivery goroutine (preserving per-key FIFO
order while distinct keys run in parallel), the bounded retry/timeout budget, and
Observer metric hooks. Services supply only the event->key+payload mapping, the
read-view Producer closure, and the per-replica transient-consumer wiring.

Reviewed pre-publish (Go + Event Sourcing + Architecture). 99% coverage, race-clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

go.sum

@@ -0,0 +1,12 @@
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