Topology-as-data: blueprint serialization & loader (C24)
Realize C24 — "the blueprint is a serializable, World-owned data value" (the topology data format), settled in docs/design/INDEX.md (commit f80571a). The blueprint becomes a value the World owns: serialized, loaded, generated, structurally searched, and reproduced — closing the graph-as-data loop (today only the blueprint -> data half exists, via model_to_json in crates/aura-engine/src/graph_model.rs). This milestone resolves the format half of #109.
Scope: the on-disk format + the data -> blueprint loader (bit-identical round-trip, C1), additive forward-compatibility, the introspectable fail-fast construction service that emits blueprints, and content-addressed run identity. Node logic stays Rust (C17); only topology becomes data.
Out of scope (separate milestones): the project-as-crate authoring layer (aura new / Aura.toml / cdylib — the #109 authoring thread) and the composable-orchestration half. The hard-wired aura-cli harnesses (HarnessKind / *_sweep_family) are scaffolding retired once those land.
