core: enforce exact public API signatures

[pzhin]

Summary

• freeze the complete pre-release public API in a checked-in signature manifest
• compare public/protected constructors, methods, fields, generic signatures, modifiers, record components, and sealed permitted subclasses
• apply the same exact check to compiled classes and the published binary JAR
• document deliberate manifest regeneration and the post-0.1.0 Revapi/japicmp baseline route
• keep the JavaDoc negative mutation classifier aligned with the exact-signature artifact gate

Verification

• rebased onto main at 96cd182; the manifest includes the new WeightDomain API and PR reports mergeable=true
• ./mvnw --batch-mode --no-transfer-progress clean verify on JDK 21: PASS (115 core tests, 5 example tests, artifact verification, coverage, and SpotBugs)
• tools/verify-javadoc-gate.sh on JDK 25: PASS
• GitHub Actions CI run #91: PASS, including JDK 17/21/25 on Ubuntu and Windows, JavaDoc negative mutation, jcstress, reproducibility, publication topology, benchmarks, and required-ci