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>

.pre-commit-config.yaml

@@ -0,0 +1,39 @@
+# See https://pre-commit.com for more information
+# See https://pre-commit.com/hooks.html for more hooks
+repos:
+- repo: https://github.com/pre-commit/pre-commit-hooks
+  rev: v6.0.0
+  hooks:
+  - id: trailing-whitespace
+  - id: end-of-file-fixer
+  - id: check-yaml
+    args:
+    - --allow-multiple-documents
+  - id: check-added-large-files
+- repo: https://github.com/alessandrojcm/commitlint-pre-commit-hook
+  rev: v9.25.0
+  hooks:
+  - id: commitlint
+    stages: [ commit-msg ]
+    additional_dependencies: [ '@commitlint/config-conventional' ]
+- repo: https://github.com/dnephin/pre-commit-golang
+  rev: v0.5.1
+  hooks:
+  - id: go-mod-tidy
+  - id: go-imports
+    args:
+    - -local
+    - gitea.unbound.se/shiny/subscriptions
+- repo: https://github.com/lietu/go-pre-commit
+  rev: v1.0.0
+  hooks:
+  - id: go-test
+  - id: gofumpt
+- repo: https://github.com/golangci/golangci-lint
+  rev: v2.12.2
+  hooks:
+  - id: golangci-lint-full
+- repo: https://github.com/gitleaks/gitleaks
+  rev: v8.30.1
+  hooks:
+  - id: gitleaks