fix: handle point-in-time temporal overlap

[starboyate]

Summary

• Treat zero-length temporal point windows as one-millisecond effective windows for overlap comparisons.
• Apply the same overlap semantics across core fallback lookup, final relation classification, JDBC stores, and MyBatis native lookup.
• Add regression coverage for same-instant point events and deterministic same-batch temporal links.

Verification

• mvn clean install
• git diff --check