feat(coven): SRP/Fernet crypto parity + multi-user coven foundation ⛧

Begin the coven evolution of cmd-chat (see docs/spec-collaborative-sandbox.md):
a Rust/ratatui client for the unchanged Python Sanic server, plus the
multi-user + zero-knowledge groundwork.

P0 — crypto parity (the spec's #1 risk), proven three ways:
- Hand-rolled SRP-6a (NG_2048, SHA-256, rfc5054 padding) matching pysrp
  byte-for-byte, incl. the fixed b"chat" SRP identity and minimal-vs-256B
  width quirks. Golden-vector unit test + offline selftest.
- Live handshake against the running server (H_AMK verified).
- Cross-language E2E: Python client decrypts a Rust-encrypted Fernet message.

P2 — multi-user coven (server):
- CMD_CHAT_MAX_USERS capacity cap (default 4, infra-for-more).
- Authoritative roster + user_joined broadcasts.
- Free the slot/username on ws disconnect (was held until 1h stale sweep).

Also: fix requirements.txt (was UTF-16, unparseable by pip).

coven/ : Rust crate (crypto.rs proven; main.rs spike CLI: selftest/handshake/srpm)
docs/  : full feature spec for the 6 requested features.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

cmd_chat/server/factory.py

@@ -24,6 +24,9 @@ def create_app(password: str = "", name: str = "cmd-chat-server") -> Sanic:
     app.ctx.ws_secret = os.urandom(32)
     app.ctx.admin_token = secrets.token_hex(16)
     app.ctx.rate_limiter = RateLimiter(max_requests=10, window_seconds=60)
+    # Coven capacity. 4 by default; raise via CMD_CHAT_MAX_USERS — infra-for-more,
+    # the cap is data not architecture (broadcast fan-out is O(N)).
+    app.ctx.max_users = int(os.environ.get("CMD_CHAT_MAX_USERS", "4"))
     app.ctx.cleanup_task = None
 
     register_lifecycle(app)