refactor: keep attr-only statements and cloned lets printable

[oinoom]

Several transforms (sink_lets, fold_let_assign, remove_redundant_let_types) were panicking whenever the rewrite engine had to reparse snippets that no longer looked like full Rust statements.

Two cases showed up when testing:

• while relocating #[derive] lines, the transform temporarily splices only the attribute text (for example just #[derive(Copy, Clone)]) while the enclosing struct body is rewritten at a higher level. Rust’s parser refuses that input, so PrintParse<Stmt> panicked before recovery finished
• newly cloned let bindings inherit the builder’s default DUMMY_SP, so pretty-printing emits an empty string even though the original code had a real initializer. The AST/text round-trip fails and subsequent rewrites abort

This PR:

• detects attribute-only snippets in PrintParse<Stmt> up front, reparses them by temporarily appending a dummy struct, and returns a StmtKind::Empty that carries the combined attribute span, which keeps the rewriter from panicking while the surrounding struct/item is printed elsewhere
• adds a helper to the AST builder that reuses the child node’s span whenever a synthesized statement would otherwise inherit DUMMY_SP. Newly cloned locals/exprs retain their original source ranges, so rewrite_at_impl can pull real snippets instead of an empty buffer