Add Maven + JUnit 5 tests and GitHub Actions CI

[xuzongbao]

Layout

Root pom.xml with:

• main sources: encryption/src (existing elven.encryption / elven.test packages stay put)
• test sources: src/test/java (outside the teaching tree so Maven does not compile JUnit as main code)

Teaching main demos are not moved into src/main/java. elven.test.test* classes still compile as part of main sources and can be run from the README javac / java commands.

JDK: Maven compile/test target is 17; GitHub Actions uses 21. Demos still run with javac on JDK 11+.

How to run tests

mvn test

CI (.github/workflows/ci.yml) runs mvn -B test on push/PR to master with Temurin JDK 21.

elven.test.test* are print-to-console mains. src/test/java/**/*Test.java are JUnit 5 assertions. Do not expect mvn test to execute the teaching mains.

Coverage

Util	Positive	Negative / extra
Base64Util	UTF-8 round-trip, known vector	null / invalid Base64
SHA256Util	deterministic 32-byte digest + lowercase hex	empty-string FIPS vector; null
HMACUtil	verify true	tampered data / MAC → false
AESUtil	encrypt/decrypt; fresh IV each time	tampered ciphertext / wrong key → AEADBadTagException
PBKDF2Util	same password+salt → same AES key	wrong password cannot decrypt AES-GCM
RSAUtil	OAEP encrypt/decrypt; PSS sign/verify	tampered ciphertext throws; tampered data/signature fail verify
DHUtil	both sides same AES-128 secret + AES round-trip	(DH param gen may take a few seconds)
ECDHUtil	same secret + AES round-trip	ECDSA verify true/false
DESUtil / DESede	light round-trip only	leftover teaching algorithms

No BouncyCastle. Crypto teaching defaults unchanged. target/ remains gitignored.