Expose caching ZK artifacts as an artifact source

[Dzejkop]

What changed

• add CachingZkArtifacts.as_zk_artifact_source
• return the exported WalletKitZkArtifactSource trait object required by UniFFI consumers

Why

Rust blanket trait implementations are not represented as Swift or Kotlin class inheritance. The generated mobile bindings therefore could construct and preload CachingZkArtifacts, but could not pass it to authenticator initializers accepting WalletKitZkArtifactSource.

Impact

Swift and Kotlin consumers can now call asZkArtifactSource() after preloading and pass the result to authenticator initialization.

Validation

• cargo +nightly fmt --all -- --check
• nix develop --command cargo check -p walletkit-core --all-features
• regenerated Swift and Kotlin UniFFI bindings and verified asZkArtifactSource() returns WalletKitZkArtifactSource

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Note

Low Risk
Small FFI surface addition with no change to ZK loading, caching, or authenticator logic beyond how types are passed across bindings.

Overview
Adds as_zk_artifact_source() on CachingZkArtifacts and EmbeddedZkArtifacts, returning Arc<dyn WalletKitZkArtifactSource> so Swift and Kotlin can pass preloaded/cached sources into authenticator setup.

UniFFI does not surface Rust blanket WalletKitZkArtifactSource impls as subclassing, so mobile code could build and preload these types but could not satisfy APIs that take the trait object until this explicit conversion exists.

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