Module structures 

The Merkle structure catalog — the sub-manifest that keeps the crate root bounded as the catalog grows (epic #30).

Invariant: adding a structure never touches src/lib.rs

Each catalog structure (Merkle Mountain Range, hash chain, sparse Merkle tree, Merkle-clock, …) is a feature-gated module declared here, in a sibling file src/structures/<name>.rs. The crate root carries a single pub mod structures; line and nothing per structure, so lib.rs stays a fixed-size manifest while this file absorbs the O(N) growth of the catalog.

Structures are reached at their path (content_addressable::structures::mmr::Mmr) and are not flattened into the frozen crate-root facade — the minimal-surface principle applied to the public API, so the root’s stable spine does not grow with the catalog.

See WORKSPACE_RULES.md → “Composition roots are manifests, not members”.

Members land here as their issues do — the first is #45 (Merkle Mountain Range), then #56 (Merkle-clock), and the rest of the Phase 2+ catalog. Until the first lands this is an intentionally empty manifest: the scaffold that lets structure #1 register with a one-line edit to this file, never the crate root.

// when #45 lands, it adds exactly one line here:
#[cfg(feature = "mmr")]
pub mod mmr;