ci: add standalone demoapp tests workflow

[rstata]

Closes the first half of #295, which is to build and test demo apps as "external" builds. (The second half — removing demoapps from the composite build — is a separate change.)

To do this, a new Standalone Demo App Tests workflow publishes all Viaduct artifacts to Maven Local and then tests each demo app in isolation — exactly as an external consumer would use them. This catches dependency-packaging bugs that composite builds hide via source substitution.

Commits

ci: add setup-build composite action and migrate build-and-test to use it

Extracts the setup-java → setup-gradle sequence into .github/actions/setup-build, a composite action with a cache-read-only input. Migrates all five jobs in build-and-test.yml to use it, removing the redundant inline steps and the actions/cache steps that were duplicating what gradle/actions/setup-gradle already manages. Downstream jobs (test, detekt, ktlint, coverage-verification) pass cache-read-only: true — only the build job needs to write the cache.

ci: add standalone demoapp tests workflow

Three-job workflow in .github/workflows/standalone-demoapp-tests.yml:

Job	Runs on	Purpose
publish-to-maven-local	ubuntu / Java 21	./gradlew publishToMavenLocal; uploads ~/.m2/repository as a workflow artifact
test-demoapps	ubuntu / Java 21 (matrix: cli-starter, jetty-starter, ktor-starter, micronaut-starter)	Downloads the artifact; runs USE_MAVEN_LOCAL=true ./gradlew test for each app in parallel
test-starwars	ubuntu × Java 17+21 (macOS available via workflow_dispatch)	Same, for the starwars demo app across Java versions

Also adds USE_MAVEN_LOCAL support to all demoapps' settings.gradle.kts files and adds the new workflow to post-slack-alerts.yml.

The default test-starwars matrix uses Ubuntu only. macOS runner provisioning added ~4 min of queue time with no meaningful benefit (Viaduct publishes platform-independent JARs). macOS can still be selected via workflow_dispatch.

fix(build-logic): clean up warnings surfaced by publishToMavenLocal

Running publishToMavenLocal carefully exposed three pre-existing issues:

• build-logic:shared applied kotlin-dsl, which internally applies java-gradle-plugin and auto-creates a pluginMaven publication. Combined with an explicit maven publication this produced duplicate publications and a spurious "no valid plugin descriptors" warning. Fixed by switching to kotlin("jvm") (shared has no Gradle API dependencies).
• test-fixtures and tools publish Gradle capability variants that cannot be expressed in Maven POM format, producing noisy metadata warnings. Suppressed with suppressPomMetadataWarningsFor().
• versioning.gradle.kts logged the resolved version at lifecycle level on every build. Downgraded to info.

perf(ci): add publishMinimal mode to speed up Maven Local publishing

Introduces a publishMinimal Gradle property that strips everything unnecessary from the publishToMavenLocal path: skips Dokka javadoc generation and sources jars, and excludes the x/javaapi modules entirely from the build graph (avoiding their heavyweight annotation processing). The standalone workflow passes -PpublishMinimal to take advantage of this.

Testing

Verified on rstata/viaduct#1:

• publish-to-maven-local completes and maven-local-repo artifact is visible in the run
• All four test-demoapps matrix shards pass in parallel
• Both test-starwars Java shards (17, 21) pass on Ubuntu
• collect-failure-info is skipped (no failures)
• Total wall time: ~6.5 min

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