Handle unused locals without scope metadata

[hunterbastian]

Fixes #96.

Summary:

• Preserve RHS expression emission when javac omits local variable metadata for an unused local assignment.
• Keep named local assignments, declarations, and newarray assignments on the existing path.
• Re-enable arbitrary-scope and runtime regression coverage for unused locals at the end of a scope.

Verification:

• Maven test-compile succeeded.
• Direct JUnit passed 6 tests: codegen ArbitraryScopeTest, codegen NullCheckTest, and runtime ArbitraryScopeTest.

Note: I did not include any payment details in this PR. Those can be handled privately after acceptance if needed.