ci: compile main sources before generating Coveralls report (#294)

* ci: compile main sources in coverage_report job

The coverage_report job was producing an effectively empty
jacocoTestReport.xml (3.4KB vs ~1.1MB locally) because no .class files
existed when coverageReportOnly ran — the job checked out source code
and downloaded .exec artifacts, but never compiled. JaCoCo's report
generator skips packages/classes it cannot resolve, so the merged XML
ended up with only <sessioninfo> entries and no <package> elements.

That made coverallsJacoco silently no-op via the
"source file set empty, skipping" branch in CoverallsReporter, so
"Push coverage to Coveralls" reported success without uploading.

Verified by downloading the coverage-report artifact from a recent run
and comparing its XML structure against a local build's report.

Assisted-By: Claude Code

* ci: share compiled classes across jobs via artifact

Eight jobs (build + 6 test + coverage_report) were each running their
own ./gradlew compile chain on the same SHA, which is wasteful and was
also the proximate cause of the silent Coveralls failure: the
coverage_report job had no .class files at all because it never ran
compileJava, so JaCoCo emitted a report with only <sessioninfo> and
zero <package> entries, and coverallsJacoco short-circuited via its
"source file set empty, skipping" branch.

Compile once in the build job, upload **/build/classes/, generated/,
and resources/ as a "compiled-classes" artifact (1-day retention),
and download it in every dependent job before running tests or
coverageReportOnly. Test jobs still invoke ./gradlew test; gradle's
UP-TO-DATE check on the unchanged .java inputs picks up the restored
outputs and skips the compile tasks. The coverage_report job now has
real bytecode for JaCoCo to map exec data against, so the Coveralls
upload actually contains source-file coverage.

Verified locally that running coverageReportOnly with no .class files
produces a 750-byte XML with only <sessioninfo>, while running it
after compileJava (or with restored classes) produces a 1.1 MB XML
with 54 <package> entries — matching the difference between CI's
broken report and a healthy local one.

Assisted-By: Claude Code

* Revert "ci: share compiled classes across jobs via artifact"

This reverts commit ecafc8c.

Author: sonus21

.github/workflows/java-ci.yaml

@@ -445,6 +445,9 @@ jobs:
       - name: Set up Gradle
         uses: gradle/actions/setup-gradle@v4
 
+      - name: Compile main sources
+        run: ./gradlew compileJava
+
       - name: Download coverage artifacts
         uses: actions/download-artifact@v4
         with: