JEP 401 and JEP 539 integrate into OpenJDK master for JDK 28

Sources: primary, OpenJDK bug database JDK-8389219, discussion
Summary: Pull request 31120 brings JEP 401, Value Classes and Objects, into the OpenJDK master branch as a preview feature, together with JEP 539, Strict Field Initialization in the JVM, which JEP 401 depends on. The bug database entry JDK-8389219 records Resolution Fixed, Status Resolved, Fix Version 28, Resolved In Build master, and a link to commit openjdk/jdk/cc278dbb. The companion issue JDK-8389220 for JEP 539 is likewise resolved. The three sub-review pull requests 31121 to 31123 close without integration by design, and Skara integrates outside the GitHub merge button, so the pull request's null merged_at is not evidence either way.
Why it matters: A mainline preview of identity-free value objects lets library authors test flattening and equality assumptions against a JDK 28 build rather than the Valhalla branch.
Follow-up: Confirm the preview ships in the JDK 28 general availability release.