Restore hydra-ext to the combined JavaDocs when it ships to Maven Central

[joshsh]

Goal

Restore hydra-ext to the published combined JavaDoc tree once hydra-ext is published to Maven Central.

Background

The historical combined JavaDoc site included both hydra-java and hydra-ext (the live docs/hydra-ext/javadoc/ tree still exists on the stale docs branch). During the 0.16.0 release hydra-ext was deliberately excluded from the regenerated combined tree, mirroring its exclusion from the Maven and PyPI publish sets: it is not in the standard sync matrix and is not published to Maven Central.

Scope

When hydra-ext is published to Maven Central:

• Add dist/java/hydra-ext/src/main/java to the combined javadoc sourcepath in the Pages build (single tree, so its cross-package @links to kernel/java resolve).
• Add hydra-ext to the publish set / dependency closure as applicable.
• Verify the previously-live hydra-ext API docs return under the published site.

Dependencies

• Blocked on hydra-ext being published to Maven Central.
• Companion to Publish combined JavaDocs from the existing Java CI build #450 (combined JavaDocs from the existing Java CI build); the same combined-tree build simply adds hydra-ext to its sourcepath once resolvable.

For #418.