meerkat-auth-core — shared auth primitives for Meerkat.
Owns the concrete implementations of the auth traits declared in
meerkat-core: TokenStore backends (File/Keyring/Auto/Ephemeral),
RefreshCoordinator impls (InMemory/FileLock), OAuth2 helpers
(PKCE/callback/device-code/token-exchange), and generic cloud-IAM
authorizers (AWS SigV4, Google ADC, Azure AD).
Non-wasm32 by construction: the filesystem, keyring, and OS lockfile
primitives this crate wraps are not available in the browser. Provider
crates (meerkat-anthropic, meerkat-openai, meerkat-gemini) depend
on this crate for their cloud-IAM backends via feature flags.
Deferral §3 B2 split (2026-04-18): extracted from meerkat-providers.