Document the standalone offline controller fix for release consumers

PR #9320 already restores the missing offline controller indexes, but the follow-up review correctly called out that the user-visible fix still needs a release note fragment under the repository's new per-change changelog layout.

Constraint: The repository no longer uses release-notes/current.yaml; release notes must be added as current/<section>/<pr-number>-<slug>.md fragments.
Rejected: Editing release-notes/current.yaml | the file has been replaced on the PR branch by fragment-based release notes.
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Keep follow-up-only PR fixes constrained to the missing release-note fragment unless a new reviewer finding requires code changes.
Tested: bash tools/hack/check-release-notes-filenames.sh; go test ./internal/provider/kubernetes -run TestNewOfflineGatewayAPIControllerIndexRegistration -count=1
Not-tested: Full repository test suite
Signed-off-by: liuhy <liuhongyu@apache.org>

Author: Aias00

release-notes/current/bug_fixes/9320-offline-controller-listenerset-indexes.md

@@ -0,0 +1 @@
+Fixed the standalone offline controller used by the File provider to register the missing ListenerSet, ListenerSet-owned route, and ClusterTrustBundle EnvoyExtensionPolicy indexes so reconciliation no longer fails with unregistered index errors.