feat(jdk-codemodel): resolve identifier symbols via javac element API

[deer]

Adds Symbol, a @Singular Trait on Identifier expressions that captures what an identifier refers to — local variable, parameter, field, type reference, this, or super — along with its declared type.

JdkExpressionConverter.visitIdentifier now calls resolveSymbol, which walks the Trees API to get the backing Element and maps its ElementKind to the appropriate Symbol variant. Five tests cover all resolved kinds.