Add JaCoCo Code Coverage (67% minimum)

[millerjp]

Summary

Adds JaCoCo code coverage with baseline threshold enforcement and CI integration.

Changes

JaCoCo Configuration:

• JaCoCo Maven Plugin 0.8.11
• Measures coverage from BOTH unit and integration tests (combined)
• Merges coverage data: jacoco.exec + jacoco-it.exec → jacoco-merged.exec
• Generates HTML report in target/site/jacoco/
• Enforces 67% line coverage minimum (current baseline)
• Build fails if coverage drops below threshold

Current Coverage (Combined):

• Line Coverage: 67% (1,123 of 1,676 lines)
• Instruction Coverage: 67%
• Branch Coverage: 53%

By Package:

• com.axonops.libre2.api: 65%
• com.axonops.libre2.cache: 67%
• com.axonops.libre2.jni: 70%
• com.axonops.libre2.metrics: 85%
• com.axonops.libre2.dropwizard: 80%
• com.axonops.libre2.util: 68%

CI Pipeline Updates:

• Renamed unit-tests → core-tests
• Stage 2 now runs: mvn verify -pl libre2-core (unit + integration + coverage)
• Uploads JaCoCo HTML report as artifact (30 day retention)
• Removed Ubuntu 24.04 x86_64 from integration stage (redundant)
• Total jobs reduced: 20 → 19

New Pipeline Structure:

Stage 1: Build JAR
Stage 2: Core Tests (unit + integration + coverage, Ubuntu 24.04) ← UPDATED
Stage 3: Integration Tests (6 platforms, skip Ubuntu 24)
Stage 4: Performance Tests (7 platforms)
Stage 5: ARM64 Final Validation (3 platforms)

Benefits

1. ✅ Enforces coverage baseline (prevents regression)
2. ✅ Combined coverage (unit + integration) for full picture
3. ✅ Fast feedback in stage 2 (~18s)
4. ✅ Coverage report available as CI artifact
5. ✅ One less redundant job (Ubuntu 24 not duplicated)

Testing

• ✅ All 450 tests passing (22 unit + 428 integration)
• ✅ Coverage check passing (67.0% ≥ 67.0%)
• ✅ CI validation in progress

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Ready to merge after CI passes