fix(build): declare the extensions.xml namespace so Renovate tracks Nisse

[jimisola]

ⓘ Follow-up to the git-derived versioning change. Small, but it is the difference between Nisse being maintained and never being updated again.

What & Why

Renovate parses .mvn/extensions.xml by default — it is in the maven manager’s default file patterns. But it rejects the file outright unless the root element carries a recognised namespace. From lib/modules/manager/maven/extract.ts:

const supportedExtensionsNamespaces = [
  "http://maven.apache.org/EXTENSIONS/1.0.0",
  "http://maven.apache.org/EXTENSIONS/1.1.0",
  "http://maven.apache.org/EXTENSIONS/1.2.0",
];

function parseExtensions(raw: string, packageFile: string): XmlDocument | null {
  ...
  if (!supportedExtensionsNamespaces.includes(attr.xmlns)) {
    return null;      // <- file silently skipped
  }

Ours declared no xmlns at all, so attr.xmlns was undefined, the file was skipped, and Nisse would have sat at 0.9.5 indefinitely while every other dependency in the repo kept moving. No warning, no PR, nothing to notice — exactly the kind of silent rot pinning a version is meant to prevent.

(0.9.5 is current as of today — released 2026-08-10. The problem is the future, not the present.)

Verification

The namespace is not just cosmetic to Maven, so I checked it does not change resolution:

git tag 9.9.9
mvn --batch-mode help:evaluate -Dexpression=project.version -q -DforceStdout
→ 9.9.9

Unchanged. CI on this PR builds the project for real.

Author checklist

• Verified against Renovate’s actual source, not just its docs
• Verified Maven still resolves the git-derived version
• Conventional Commit title, DCO sign-off