Fix parser reference expression edge cases

[grian32]

Problem

parseReference parses a full expression after & but only stores it if the result is an identifier. For any other expression, ReferenceExpression.Var remains nil and later String/Position can panic.

Tasks

• Decide whether & is restricted to identifiers or supports broader lvalues.
• If restricted, emit a parser/checker error for non-ident references.
• If broader, change AST to store an expression/lvalue.
• Add tests for &(x), &p.field, &arr[i], and invalid cases.

Refs: parser/expr_prefix.go:21, parser/ast.go:328.

Related: #4