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.mod

@@ -0,0 +1,14 @@
+module gitea.unbound.se/shiny/subscriptions
+
+go 1.25
+
+require (
+	github.com/google/uuid v1.6.0
+	github.com/stretchr/testify v1.11.1
+)
+
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)