core,nix: Add OnnxSession + pinned model weights behind inference feature

The real Detector and Embedder implementations need ONNX Runtime; this
commit wires up the dependency, the Nix devShell, the pinned model
weights, and a thin session wrapper they'll build on top. The actual
SCRFD detector and ArcFace embedder land in subsequent sub-milestones
(M3.3, M3.5).

Feature shape mirrors v4l2: `pareidolia-core` keeps `inference` opt-in
(default = []); the daemon enables it via its own default-on `inference`
feature that proxies to core. CI's --no-default-features keeps libv4l +
libonnxruntime out of the Nixery image while normal builds get the full
pipeline.

`ort`'s `load-dynamic` feature is the cleanest fit for Nix: the crate
dlopens libonnxruntime at runtime via ORT_DYLIB_PATH rather than linking
against it at build time. The devShell wires ORT_DYLIB_PATH to nixpkgs's
`.so` so both build and run work without bundling C++ libs.

`OnnxSession` exposes the underlying `ort::Session` directly rather
than wrapping every ort API behind a Pareidolia-flavoured alternative —
chasing ort's API as it evolves is more churn than the abstraction is
worth. The boundary is at error types: `ort::Error` is mapped to
`PipelineError::Backend` so the rest of the pipeline stays
backend-agnostic.

Model weights: the M6 plan had Nix-driven model fetching land alongside
the NixOS module, but the M3 sub-milestones need real ONNX files for
their hardware tests, so the fetching infrastructure moves forward.
`nix/models.nix` pins SCRFD-10G and ArcFace glintr100 from the
`immich-app/buffalo_l` HuggingFace mirror (a maintained mirror of the
InsightFace bundle, stable since 2023). The devShell exports
PAREIDOLIA_TEST_{MODEL,SCRFD,ARCFACE} pointing at the materialised store
paths so `just test-hardware` runs end-to-end with no manual setup.

The pinned weights are also exposed as flake packages
(`nix build .#scrfd-10g`) for ad-hoc use outside the devShell.