Deterministically box cyclic cddl-derive struct references

[Copilot]

cddl-derive generated direct struct-to-struct references for cyclic CDDL rules, which produces infinite-size Rust types (e.g. Node ↔ Child). This change introduces deterministic cycle breaking by boxing recursive edges during code generation.

• Recursive type graph analysis

  • Build a reference graph across generated type defs.
  • Compute strongly connected components (SCCs) to identify cyclic groups (including self-recursion).

• Deterministic cycle breaking

  • For struct fields that reference types in the same cyclic SCC, mark selected edges as boxed using a stable lexical ordering rule.
  • Ensures one side of a mutual cycle is boxed consistently, avoiding nondeterministic output.

• Codegen output updates

  • Add boxed field support in RustField (is_boxed).
  • Emit Box<T> / Option<Box<T>> for marked recursive fields while preserving existing optional/serde behavior.

• Regression coverage

  • Add tests for:
    • mutual recursion (node/child)
    • self recursion (node containing node)

node = { ? "child": child }
child = { ? "parent": node }

Now generates finite Rust types (deterministically), e.g.:

pub struct Node {
    pub child: Option<Box<Child>>,
}

pub struct Child {
    pub parent: Option<Node>,
}