Java 25 Baseline

This release establishes Java 25 as the baseline for all design pattern examples.

Key changes:
- All 21 Maven projects now compile with Java 25
- Fixed class name/filename mismatches in observer patterns
- Added GitHub Actions parallel build workflow (3 jobs, ~36 second build time)
- Added build status badge to README

Build Status: All 21 projects compile successfully
Sections: Creational (5), Structural (5), Behavioral (11)

Update Spring Boot REST APIs course title in README (#4)