Link javadoc to this BOM's javadoc union

Every component managed by pom-scijava has its javadoc on
javadoc.scijava.org, unioned per pom-scijava release into a single
-link target. Projects extending pom-scijava now link to the union of
the release they build against, so classes of any managed dependency
are linked from their javadoc, with one element-list fetch instead of
one per dependency.

The URL uses ${project.parent.version}, which is interpolated in the
child project: it is this BOM's version when the project extends it
directly. Others (e.g. modules of a multi-module build) can override
scijava.javadoc.url.bom. A version the site does not have, such as a
locally installed 999-test or a release not yet published there,
redirects to the newest union; the javadoc tool follows the redirect
with a warning, and its links keep the original URL, which resolve
exactly once that release is on the site. Setting the property empty
drops the link, e.g. to build javadoc offline.

Closes #130.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: ctrueden

pom.xml

@@ -273,6 +273,25 @@
 		<license.licenseName>N/A</license.licenseName>
 		<license.copyrightOwners>N/A</license.copyrightOwners>
 
+		<!--
+		Javadoc of every component managed by this BOM, unioned into a single
+		link target for the maven-javadoc-plugin: classes of any managed
+		dependency are linked from the generated javadoc, at the versions this
+		BOM release manages, with one fast lookup instead of one per dependency.
+		See https://github.com/scijava/javadoc.scijava.org.
+
+		NB: This is interpolated in your project, where ${project.parent.version}
+		is your parent's version: this BOM's, if your project extends it directly.
+		If it does not (e.g. a module of a multi-module build), override this
+		property with the pom-scijava version you build against. A version the
+		site does not have (e.g. a locally installed 999-test, or a release not
+		yet published there) falls back to the newest one.
+
+		To build javadoc without contacting javadoc.scijava.org (e.g. offline),
+		drop the links: -Dscijava.javadoc.url.bom= -Dscijava.javadoc.url.java=
+		-->
+		<scijava.javadoc.url.bom>https://javadoc.scijava.org/org.scijava/pom-scijava/${project.parent.version}/</scijava.javadoc.url.bom>
+
 		<!-- Managed dependency versions - SciJava component collection -->
 
 		<!--
@@ -7215,6 +7234,22 @@
 		</dependencies>
 	</dependencyManagement>
 
+	<build>
+		<pluginManagement>
+			<plugins>
+				<plugin>
+					<artifactId>maven-javadoc-plugin</artifactId>
+					<configuration>
+						<!-- Link to this BOM's javadoc union; see scijava.javadoc.url.bom. -->
+						<links combine.children="append">
+							<link>${scijava.javadoc.url.bom}</link>
+						</links>
+					</configuration>
+				</plugin>
+			</plugins>
+		</pluginManagement>
+	</build>
+
 	<profiles>
 		<profile>
 			<id>windows</id>