fix(auth): fall back for Turtle security extensions

[paleophyte]

Summary

• My Turtle WoW server really doesn't want to accept auth protocol-3.
• Keep Turtle's configured auth protocol preference at 3, but treat v8 security-extension flags on a protocol-3 challenge as a protocol mismatch so the UI fallback retries protocol 8.
• Stop writing a v8 security-flags byte in the legacy LOGON_PROOF packet shape.
• Add a regression check for legacy vs v8 proof packet sizes.

Why

Turtle can advertise PIN/security extensions even when the regular client starts with the vanilla-family protocol-3 preference. Previously the client sent a doomed legacy proof after seeing those v8-only flags, producing a login failure before fallback could happen.

Validation

• cmake --build build --target test_realm_list --config Debug
• ./build/bin/test_realm_list.exe
• WOWEE_LOG_LEVEL=debug ./build/bin/wowee

...
[2026-07-15 17:23:17.036] [DEBUG] Handling LOGON_CHALLENGE response
[2026-07-15 17:23:17.036] [INFO ] LOGON_CHALLENGE response: Success
[2026-07-15 17:23:17.036] [DEBUG] Parsed LOGON_CHALLENGE response:
[2026-07-15 17:23:17.036] [DEBUG]   B size: 32 bytes
[2026-07-15 17:23:17.036] [DEBUG]   g size: 1 bytes
[2026-07-15 17:23:17.036] [DEBUG]   N size: 32 bytes
[2026-07-15 17:23:17.036] [DEBUG]   salt size: 32 bytes
[2026-07-15 17:23:17.036] [DEBUG]   Security flags: 1
[2026-07-15 17:23:17.036] [DEBUG]   PIN grid seed: 2613764122
[2026-07-15 17:23:17.036] [WARN ] Server sent security flags: 0x1
[2026-07-15 17:23:17.036] [WARN ]   PIN required
[2026-07-15 17:23:17.036] [INFO ] Challenge: N=32B g=1B salt=32B secFlags=0x1
[2026-07-15 17:23:17.036] [ERROR] Authentication failed: Server requires auth protocol 8 security extensions
[2026-07-15 17:23:17.036] [DEBUG] Auth state: 2 -> 10
[2026-07-15 17:23:17.037] [INFO ] Auth failed on protocol 3 (Server requires auth protocol 8 security extensions) - retrying with protocol 8
...
[2026-07-15 17:23:19.315] [INFO ] Server proof verified successfully!
[2026-07-15 17:23:19.315] [DEBUG] Auth state: 6 -> 7
[2026-07-15 17:23:19.315] [INFO ] ========================================
[2026-07-15 17:23:19.315] [INFO ]    AUTHENTICATION SUCCESSFUL!
[2026-07-15 17:23:19.315] [INFO ] ========================================