Apple has deprecated the legacy /appleauth/auth/signin endpoint and
now blocks it, causing "Invalid Session Token" errors for all users
when their trust token expires. The browser login flow now requires
SRP (Secure Remote Password), a cryptographic handshake that never
transmits the password.
Replace Session.SignIn() with a multi-step SRP-6a flow:
1. authStart - initialize session at /authorize/signin
2. authFederate - submit account name to /federate
3. authSRPInit - exchange client public value for salt/B at /signin/init
4. authSRPComplete - send M1/M2 proofs to /signin/complete
The SRP implementation uses the RFC 5054 2048-bit group with SHA-256
and Apple's NoUserNameInX variant. Password derivation supports both
s2k and s2k_fo protocols via SHA-256 + PBKDF2.
The 2FA and trust token flow is unchanged. Auth headers for all
idmsa.apple.com requests now include X-Apple-Auth-Attributes,
X-Apple-Frame-Id, and use Origin/Referer of https://idmsa.apple.com.
Fixes#8587
35e4f60548