Pure helpers for computing auth session expiry timing.

Used by the adapter layer (T94/T95) to decide when to schedule an :auth_expired re-auth timer. TTL sources in priority order:

1. Response-level — auth response returns {:ok, %{ttl_ms: N}} from handle_auth_response/2 (e.g. deribit emits expires_in in seconds)
2. Config-level — auth_config[:auth_ttl_ms] as a static override
3. None — no TTL source, no timer scheduled (caller should not re-auth on a clock; rely on server-driven auth-failed errors instead)

Not a CCXT.WS.Auth.Behaviour implementer — this is a pure utility shared across patterns that return TTL metadata.

Example

auth_meta = %{ttl_ms: 900_000}
auth_config = %{pattern: :jsonrpc_linebreak}

ttl_ms = Expiry.compute_ttl_ms(auth_meta, auth_config)
# => 900_000

Expiry.schedule_delay_ms(ttl_ms)
# => 720_000  (80% safety margin)