feat(protocol): expose research lineage — Versions, Branches, Forks and Proposals — through Hub discovery #1748

Problem

GraphForge has durable research lineage: immutable Versions, Branches with current heads, Forks with origin citations, Proposals, and Branch creation from frozen Slices (ADRs 0039–0044, #1352, #1356, #1357). #1357 defines consumer-neutral references and leaves "URLs and hosting" to associated projects. Hub discovery (#906, extended by #1732) still exposes only complete Project versions, so a Hub can't show or clone any of this lineage.

Objective

Extend the Rust-owned discovery contract with read-only research lineage, so a conforming Hub can render Versions, Branches, Forks and Proposals and link them, and gf clone can fetch an exact Branch head or Version.

Requirements

• Advertise published research Versions and Branch heads for a repository. Discovery refs map to Branch heads; immutable Version references never follow a later head (ADR 0044).
• Expose Fork origin citations (original Project identity and source Version) and Branch genealogy (parent, base Version, ultimate Project lineage) as typed references with exact identities.
• Expose Slice provenance only as a Branch creation selection commitment. No persistent Slice entity (ADR 0040).
• Expose published Proposal packages as frozen, selected projections with their source Branch/Version identity; review decisions remain destination-local.
• Clone or import an exact Branch head or Version through the existing research interchange component, with the same verification, atomicity and network-safety rules as gf clone.
• Keep package, transport, Project, Version and projection identities distinct. A redacted or partial projection never masquerades as the complete Version.
• Unknown required research capabilities fail closed with unsupported_future before object reads or project mutation.
• Update the JSON Schema and conformance corpus with a fixture that has a Fork, two Branches and a Proposal.

Acceptance

• A consumer can list a repository's Branches and Versions, resolve "forked from" to the origin Project identity, and clone a Branch head and an immutable Version into distinct destinations that reopen with identical lineage.
• A partial projection is reported as a projection of its source Version, not as that Version.

Non-goals

• Publishing or uploading (feat(cli): publish Projects, Branches and Forks to a Hub #1749), Proposal submission across owners, and review/acceptance on a Hub.
• Hub UI, accounts and access enforcement.

Related

#906, #1732, #1352, #1356, #1357, ADRs 0039–0044; CurateLabs/graphforge-nextjs M3.