Fix skin cannot be null by declaring the fluido skin in site.xml

[ascheman]

Problem

The master Verify build has been red since late June: every run fails at the site step with

Failed to execute goal org.apache.maven.plugins:maven-site-plugin:3.22.0:site (default-site):
skin cannot be null

The site 2.0.0 descriptor (SITE/2.0.0) no longer supplies a default skin, so maven-site-plugin 3.22.0 requires an explicit <skin> element, and src/site/site.xml doesn't declare one. (The parent's skin is declared only in maven-parent's own site.xml; it is not published as an inheritable site-descriptor artifact — -X shows No parent level 1/2/3 site descriptor — so it never reaches this project.)

This single failure blocks the whole repo: since master is red, every open PR inherits a red CI and nothing can be merged.

Fix

Declare maven-fluido-skin explicitly in site.xml — five lines, no source or dependency changes.

Why this is the minimal change

The -P run-its verify build itself (including the reproducible IT that deploys via file://) is already green on 4.0.0-beta-3 — the site skin is the only thing failing. I verified the skin-only change on a fork: the full Verify matrix passes 8/8 (ubuntu/macos/windows x JDK 17/21/25) against 4.0.0-beta-3 — https://github.com/aschemaven/maven-source-plugin/actions/runs/29831325551.

This is intentionally the minimal subset needed to unblock CI. It overlaps with the site-skin part of #309, but without that PR's IT-plumbing/transport changes, which aren't needed to make the build green on beta-3.

Testing

mvn clean site and the full -P run-its verify matrix pass on 4.0.0-beta-3 (fork run linked above).

[Copilot]

Pull request overview

This PR unblocks the repository CI by fixing Maven Site generation failures caused by the SITE/2.0.0 descriptor no longer providing a default skin, which makes maven-site-plugin:3.22.0 require an explicit <skin> in src/site/site.xml.

Changes:

• Declare org.apache.maven.skins:maven-fluido-skin:2.1.0 explicitly in the project site descriptor (src/site/site.xml).

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[ascheman]

For the record: the underlying cause — that a missing <skin> in a site 2.0.0 descriptor produces a cryptic NullPointerException: skin cannot be null rather than an actionable error — is now filed upstream as apache/maven-site-plugin#1286 (asking for a clear message, or a restored default). This PR is the local fix (declare the skin explicitly).

[slawekjaranowski]

can be as workaround, please add a comment that should be check with next Maven version

We have it in parent site descriptor ... but here parent is not resolved

It was fixed in Maven core ... I don't remember exactly which issue ...

[ascheman]

Thanks @slawekjaranowski! Verified your point: on 4.0.0-rc-5 / 4.0.x-SNAPSHOT / 4.1.0-SNAPSHOT the parent site descriptor is resolved and the parent's skin is inherited — mvn site is green with no local <skin>. It only fails on 4.0.0-beta-3 (which this plugin still builds against), where the parent descriptor isn't resolved (-X: No parent level N site descriptor).

I've added a comment in site.xml marking this as a beta-3 workaround, to be dropped once the build moves to a newer Maven (the #268 / RC-6 track). I couldn't pin the exact core fix issue — if you recall it, happy to reference it.

[github-actions]

@ascheman Please assign appropriate label to PR according to the type of change.

[slawekjaranowski]

Probably it was:

• [MNG-8172] Fix site building maven#1594