Further refine architecture documentation

docs/ARCHITECTURE.md

@@ -108,14 +108,14 @@ Client                                    Server
   │  WASM: generate Ed25519 keypair          │
   │  private key → thread_local storage      │
   │                                          │
-  ├─── { public_key: hex } ────────────────►│
+  ├─── { public_key: hex } ─────────────────►│
   │                                          │  Generate session_id (32 bytes CSPRNG)
   │                                          │  Generate salt₀ (16 bytes CSPRNG)
   │                                          │  H₀ = Blake3(session_id ║ pub_key ║ salt₀)
   │                                          │  Generate random VM program (8–16 opcodes)
   │                                          │  Store: session_id, pub_key, salt₀, H₀
   │                                          │
-  │◄── { session_id, salt, opcodes, H₀ } ───┤
+  │◄── { session_id, salt, opcodes, H₀ } ────┤
   │                                          │
   │  Store: session_id, prevHash=H₀,         │
   │         currentSalt=salt₀, opcodes       │