fix(imperative-codemodel): distinguish bare return from return null

[deer]

Return.expression was non-optional, forcing JdkStatementConverter to substitute NullLiteral for bare return; statements. This made void returns indistinguishable from explicit return null; in the captured AST.

Return.expression is now Optional<Expression>. Return.of(CodeModel) is added for void returns, and Return.of(Expression) continues to serve value-returning statements. visitReturn passes Optional.empty() for bare returns instead of a fabricated null literal. Marshalling updated to Optional<Marshalled<Expression>> throughout.

New test shouldCaptureBareReturnAsEmptyOptional verifies the fix. Five existing callsites updated to .expression().orElseThrow().