Add Scalpel for incremental CI builds

[jamesnetherton]

Registers Scalpel as a Maven core extension (disabled locally by default via .mvn/maven.config) and wires it into the CI pipeline so that pull requests only build and test the modules affected by their changeset.

Changes:

• .mvn/extensions.xml: register eu.maveniverse.maven.scalpel:extension:0.3.4
• .mvn/maven.config: disable Scalpel by default (-Dscalpel.enabled=false)
• .github/workflows/ci-build.yaml:
  • Generate a Scalpel report during the examples build (mode=report, reportFile written outside target/ so clean does not delete it)
  • Fetch the PR base branch before the build so Scalpel can find the merge base in the shallow CI clone
  • Skip the fetch (and fall back to a full build) when the PR carries the ci/disable-incremental label
  • Replace the Groovy-based matrix with generate-test-matrix.sh
  • Add 'modules' output consumed by JVM and Windows test jobs via -pl
• .github/generate-test-matrix.sh: new script replacing generate-test-groups.groovy; reads the Scalpel JSON report, distributes single-module projects round-robin across up to 10 native test groups, gives multi-module projects a dedicated group, and expands multi-module paths for the -pl modules list

[jamesnetherton]

I did some testing on my fork and it was working as expected.

If we hit issues post merge we can either fix forwards or roll the change back.

[jamesnetherton]

I'll merge this, then follow up with a PR for testing. Can't test in this PR, because Scalpel considers the .mvn directory for inclusion in its full build triggers.