chore(build): harden Maven Central release flow

[sklarsa]

Summary

• clarify the SNAPSHOT-tag failure message in the release workflow
• keep duplicate Maven Central publishes as real failures instead of silently skipping them
• document the release flow in README and release docs, including not tagging before release:prepare and not running local deploys during the normal Actions-owned path

Validation

• python3 YAML parse for .github/workflows/maven_central_release.yml
• git diff --check