test(io): Synfire registry model compatibility (pinned fixtures → nir-rs load)

[rmems]

Current status

This is the active Synfire → nir-rs compatibility baseline. It was previously marked Done without vendored Synfire fixtures; the current repository still records that Synfire pulls remain tracked here. It is reopened for implementation and verification.

Synfire is the first interoperability path. It proves that nir-rs can read public NIR artifacts; it does not add an axon-encoder exporter or convert a Spikenaut-SNN Q8.8 .mem bank.

Scope

• Pull, pin, and vendor:
  • pabogdan/lifneuron:1.0.0 — minimal LIF graph.
  • pabogdan/ifsynfire:0.1.0 — synfire-chain topology.
  • pabogdan/nmnistcnn:1.0.0 — convolutional event-vision graph.
• Record source, exact version, SHA-256, retrieval date, node count, and edge count in a fixture manifest.
• Add feature-gated HDF5 tests that load each vendored artifact, validate graph structure, and assert recorded topology.
• Document the maintainer-only refresh command: synfire pull <org>/<model>:<version>.

Constraints

• Default tests and CI use committed fixtures only: no network, Synfire authentication, or live registry access.
• Compatibility failures need a model/version/error-class record; no silent skips.
• This phase does not add axon-encoder::to_nir(), publish Limen models to Synfire, run full neuromod simulation, or create the deferred Spikenaut conversion issue.

Relationships

• Parent: LIM-822.
• Related fixture work: LIM-999 and LIM-1086.
• Future consumer/exporter context: LIM-831 and RM-382.

Acceptance

• All three pinned artifacts and complete checksum/topology manifest are committed.
• HDF5 fixture tests load and validate each artifact in CI without live registry access.
• Refresh and review procedures are documented.
• Unsupported artifacts get explicit follow-up tracking.

[linear-code]

LIM-1085

[rmems]

Tracker correction (2026-09-20): reopened because the intended Synfire fixture suite is not present in the current tree. This is now the active source of truth for the three pinned, hermetic Synfire compatibility fixtures under the existing nir-rs v0.4.4 milestone. The historical axon-encoder exporter record remains separately preserved at Limen-Neural/axon-encoder#43.