SPIKE: Protocol 28 (CAP-0084) — nightly-next core image handoff

Comment-only handoff. Real nightly-next bump (protocol 28 + CAP-0084 core ref)
deferred until a -vnext stellar-core image is published (stellar-core#5337).

Author: sisuresh

.github/workflows/build-testing.yml

@@ -4,6 +4,12 @@ name: Testing
 # versions of software. That means each ref should only ever be specified as a
 # version tag for software that's either the latest release candidate or latest
 # release.
+#
+# CAP-0084 (Muxed Contract Addresses) / Protocol 28 handoff:
+# The nightly-next quickstart image bump (protocol_version_default -> 28 and the
+# core ref -> the CAP-0084 SPIKE build) is DEFERRED until a `-vnext` stellar-core
+# image is published. Track the core build at stellar/stellar-core#5337. Once a
+# published image exists, bump the nightly-next core ref + protocol default here.
 
 on:
   workflow_call: