Add Java educational track through advanced modules

[lucasbellesi]

Summary

• Add Java 21 educational content for foundations and core modules, with examples, exercises, output contracts, and docs.
• Add Java foundations/core projects and assessments with smoke coverage.
• Start Java 03-advanced with structs/classes and constructors/invariants modules.
• Extend automation for Java entrypoints, partial module overrides, contracts, smoke checks, and CI/tooling docs.

Validation

• python scripts/automation.py lint
• python scripts/automation.py smoke-languages
• python scripts/automation.py verify-repo

verify-repo passed locally with Java compilation and output contracts.