Rust traits/types representation: encode arbitrary Rust values and type shapes as links

[konard]

Context

Issue #47 requires "also Rust traits/types representation" as a storage form.
LinkType::Object and LinkNetwork::insert_object cover identity and
circular references, and lino-objects-codec fixtures are gated, but arbitrary
user structs/enums cannot be encoded to links and decoded back, and Rust type
shapes are not represented. See requirements.md
R-9 and solution-plans.md S-7.

Ecosystem parity target: lino-objects-codec 0.2.1 (Rust) / 0.4.0 (npm) -
object encode/decode with identity and circular-reference preservation
(formats-storage-apis.md Part B).

Blocked by: #05 (the LiNo serializer makes encodings inspectable and
provides the codec's text form).

Scope

• ToLinks / FromLinks traits with implementations for primitives, Vec,
  Option, maps; decide between a #[derive(Links)] proc-macro and a serde
  Serializer/Deserializer adapter (serde reuses the whole ecosystem at zero
  macro cost) - record the decision and rationale.
• Preserve shared references and cycles via object-identity links, porting
  lino-objects-codec's shared-reference and circular-reference cases as
  fixtures.
• Represent Rust type declarations (struct/enum/trait shape) as links using
  the existing self-description roots (type, Type, field) so type shape
  is queryable data.

Acceptance criteria

• A user struct with nesting, sharing, and a cycle round-trips through
  links and through LiNo text.
• Type-shape links are queryable via LinkQuery.
• lino-objects-codec parity fixtures extended to the new codec path.
• Changelog fragment added (bump: minor).

References

• Requirement: requirements.md R-9
• Solution: solution-plans.md S-7
• Part of We need to carefully compare all competitors in all scopes with out project, and make sure we have the richest features set #47; work lands on branch issue-47-76af108c0f24 (PR Finish issue #47 parity feature set #48).

---

Filed from docs/case-studies/issue-47/proposed-issues/07-rust-types-links-codec.md. Part of the implementation plan for #47.