Share build logic through convention plugins and enable the configuration cache - #17

Gradle build refactoring following Gradle best practices. Based on #16 (CI improvements); review only the top commit.

What changes

• build-logic/ included build (as in the Apache Pulsar build, kept small) with four precompiled convention plugins:
  • jonoffcpu.java-conventions: Corretto 25 toolchain, --release from jonoffcpu.javaRelease (default 21; the agent sets 17; it also drives the published org.gradle.jvm.version), UTF-8, Javadoc options, sources/javadoc JARs, disabled JUnit test, and check → every FixtureExec + :spotlessCheck (by path).
  • jonoffcpu.publish-conventions: the shared Maven Central/POM setup; modules set jonoffcpuPublication.displayName/description, and the converter overrides the license.
  • jonoffcpu.shaded-jar-conventions: the embeddedRuntime configuration, verifyDependencyDigests, the shaded JAR and its publication; modules add relocations and contents.
  • jonoffcpu.protobuf-conventions: the lite codec from docs/schema.
• Typed tasks with declared inputs replace ad-hoc doLast blocks that called build-script functions: VerifyDependencyDigests, VerifyJarContents, VerifyNativeLibraries, GenerateNativeChecksums, VerifyAgentJar, and FixtureExec, which replaces the root script's STARTED/PASSED logging. The host C library is detected in a ValueSource, so /proc/self/maps (different on every run) is no configuration input.
• Version catalog gradle/libs.versions.toml holds every library and plugin version. Repositories are declared once in settings, with FAIL_ON_PROJECT_REPOS.
• No cross-project configuration: the root subprojects {} block is removed, the root directory is read via isolated.rootProject, and :spotlessCheck is depended on by path.
• gradle.properties: org.gradle.configuration-cache=true, org.gradle.configureondemand=true and org.gradle.parallel=true, next to org.gradle.caching=true from Speed up CI and make its builds visible #16.
• Spotless pinned to LF line endings. Its default reads git attributes through JGit, whose file-system probes made the configuration cache miss on every run ("an input to unknown location has changed"). Only gradlew.bat is CRLF, and Spotless doesn't format it.
• Docs: AGENTS.md's build rules (catalog, convention plugins, configuration-cache rules) and the README's build notes and repository layout.

Verification

• Configuration cache: before, the check graph stored with 22 problems; now it stores with none and a second run prints Reusing configuration cache. The same holds for publishToMavenLocal and a dry run of publishAndReleaseToMavenCentral -PsignAllPublications=true.
• Configure-on-demand: :jonoffcpu-jfr-converter:check configures only :, build-logic and :jonoffcpu-jfr-converter.
• Build cache: after clean, compileJava, generateProto, javadoc and the correlator's shadowJar are FROM-CACHE.
• Checks: spotlessCheck :jonoffcpu-agent:check :jonoffcpu-correlator:check :jonoffcpu-jfr-converter:check -PnativeArchitectures=x86_64 -PnativeLibcs=all passes; all 19 fixture tasks run, including the native ones.
• Artifacts compared with Speed up CI and make its builds visible #16's build (publishToMavenLocal, x86-64 glibc+musl): identical JAR entry lists, POMs and Gradle module metadata. The only difference is the order of the shaded JARs' manifest attributes, hence their size and hashes. Published JVM versions are unchanged: agent 17, correlator and converter 21.
• The CI package job's command (-PprebuiltNative=true -PnativeArchitectures=all -PnativeLibcs=all) plans no Docker builds, as before.

Not verified locally

• Actual signing and upload in the release workflow, which needs the release secrets. The release graph stores and reuses under the configuration cache without problems; if in doubt, the release step can add --no-configuration-cache.
• arm64, which is left to CI.