Nix store abstraction with SeaORM metadata.

Provides the [Store] trait that all store backends implement (local filesystem, binary cache over HTTP). Includes SeaORM entity models that map 1:1 to the Nix SQLite schema.

Architecture

• [Store] — async trait defining the store contract (query, add, GC)
• [LocalStore] — reads /nix/store + SQLite via SeaORM
• [BinaryCacheStore] — read-only HTTP client for cache.nixos.org / Cachix / Attic
• [HttpClient] — pluggable HTTP backend for testability

Error handling

All store operations return [StoreResult<T>], which wraps [StoreError]. Binary cache operations produce [BinaryCacheError] internally, which converts into [StoreError] via From impls.