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>

coven/tools/gen_vectors.py

@@ -0,0 +1,32 @@
+import srp, srp._pysrp as p, hashlib, binascii
+srp.rfc5054_enable()
+# force pure-python to match (and check active backend)
+import srp as S
+print("# backend:", S._mod.__name__)
+H=hashlib.sha256
+user=b"chat"; pw=b"labtest"
+salt=bytes.fromhex("0a1b2c3d")             # fixed 4-byte salt
+a=bytes.fromhex(("11"*256))                # fixed 256-byte a (high bit set via 0x11.. ok? need high bit) 
+a=bytes([0x80])+bytes.fromhex("22"*255)    # ensure high bit set, 256 bytes
+b=bytes([0x80])+bytes.fromhex("33"*255)
+# verifier from known salt: replicate create_salted_verification_key internals with fixed salt
+N,g=p.get_ng(p.NG_2048,None,None)
+x=p.gen_x(H, salt, user, pw)
+v=pow(g,x,N)
+v_bytes=p.long_to_bytes(v)
+usr=p.User(user,pw,hash_alg=p.SHA256,ng_type=p.NG_2048,bytes_a=a)
+I,A=usr.start_authentication()
+svr=p.Verifier(user,salt,v_bytes,A,hash_alg=p.SHA256,ng_type=p.NG_2048,bytes_b=b)
+s2,B=svr.get_challenge()
+M=usr.process_challenge(salt,B)
+HAMK=svr.verify_session(M)
+usr.verify_session(HAMK)
+def hx(x): return binascii.hexlify(x).decode()
+print("N_bits", N.bit_length())
+print("salt", hx(salt))
+print("A", hx(A))
+print("B", hx(B))
+print("M", hx(M))
+print("HAMK", hx(HAMK))
+print("K", hx(usr.K))
+print("authok", usr.authenticated())