feat(srp): pure-Python server Verifier so Termux can HOST a room

The srp package's C-extension has no aarch64/bionic wheel, so a phone in
Termux couldn't run the room server. Add the server side of SRP-6a
(create_salted_verification_key + Verifier) to the pure-Python shim and
fall srp_auth.py back to it on ImportError.

Proven live: a laptop (C-ext srp client) authenticated against a room
HOSTED ON THE PHONE (pure Verifier) over Tailscale — cross-implementation
handshake with matching session keys. Locked in with interop tests
covering real-client-vs-pure-Verifier, pure-vs-pure, and a wrong-password
negative control.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cmd_chat/server/srp_auth.py

@@ -3,7 +3,10 @@ from dataclasses import dataclass, field
 from typing import Optional
 from uuid import uuid4
 
-import srp
+try:
+    import srp
+except ImportError:  # no aarch64 wheel / C-ext build (Termux) — use the shim
+    from ..client import _srp_pure as srp
 
 
 srp.rfc5054_enable()