Operator Assignment Support

[rcosta358]

Description

This PR adds support for Java operator assignments such as +=, -=, *=, /=, and %= in the refinement checking.
It works by modeling operator assignments as equivalent simple assignments: x += y → x = x + y.

Example

@Refinement("_ > 0")
int plus(@Refinement("_ >= 0") int x) {
    x += 1; // x is now > 0
    return x;
}

Before we had an error in the return statement because the x += 1 was ignored. Now we don't.

Related Issue

None.

Type of change

• Bug fix
• New feature
• Documentation update
• Code refactoring

Checklist

• Added/updated tests under liquidjava-example/src/main/java/testSuite/ (Correct* / Error*)
• mvn test passes locally
• Updated docs/README if behavior or API changed