Feat: ConnAuth HMAC verification gated by YAMA_CONN_AUTH_KEY

Verify the HMAC-SHA256 signature and timestamp of device sub-connections
(screen/terminal/file) when the YAMA_CONN_AUTH_KEY environment variable is
set. With the key set, a sub-connection whose signature or clock skew fails
is answered with a non-OK ConnAuth status and dropped; without the key the
legacy always-OK behavior is preserved.

The key is never compiled in: it is read from the environment at startup, so
no secret enters the repository. Source-IP binding in hub.BindScreenConn
remains in place as defense-in-depth.

Co-Authored-By: deepseek-v4-pro

server/go/protocol/commands.go

@@ -140,11 +140,26 @@ const (
 	ConnAuthOffClientID = 1 // uint64 LE — pin to the sub-conn so later
 	//                       // 1-byte tokens (TOKEN_TERMINAL_START etc.) can
 	//                       // resolve the parent device.
+	ConnAuthOffTimestamp  = 9  // uint64 LE - Unix seconds, anti-replay window
+	ConnAuthOffNonce      = 17 // [16]byte - part of the signed message
+	ConnAuthOffSignature  = 33 // [64]byte - hex of HMAC-SHA256(key, clientID||timestamp||nonce)
+	ConnAuthSignatureSize = 64
+
 	// ConnAuthAck field offsets within the outbound 256-byte buffer.
 	ConnAuthAckOffStatus     = 1 // uint8
 	ConnAuthAckOffServerTime = 2 // uint64 LE
-	// Status codes.
-	ConnAuthStatusOK byte = 0
+	// ConnAuthTimestampToleranceSec bounds |server - client| clock skew in
+	// seconds (matches CONN_AUTH_TIMESTAMP_TOLERANCE_SEC in common/commands.h).
+	// A timestamp outside this window is treated as replayed or forged.
+	ConnAuthTimestampToleranceSec = 300
+
+	// Status codes (match common/commands.h::ConnAuthStatus). The client treats
+	// any non-zero status as failure and drops the sub-connection.
+	ConnAuthStatusOK            byte = 0
+	ConnAuthStatusBadSize       byte = 1 // packet shorter than the signature field
+	ConnAuthStatusClockSkew     byte = 2 // timestamp outside tolerance
+	ConnAuthStatusBadSignature  byte = 3 // HMAC mismatch
+	ConnAuthStatusInternalError byte = 4
 )
 
 // CMD_MASTERSETTING is the server's reply to a fresh client login. The