Feat: One-way TLS transport for the MFC server

Add optional one-way TLS to the MFC server (server/2015Remote) so the device
port can serve both TLS clients and legacy plaintext clients on the same port,
mirroring the Go server.

Server: on a connection's first bytes, sniff for a TLS ClientHello (0x16 0x03
+ handshake type 0x01). When [tls] Enabled=1 and the identity loads, hand the
socket to a short-lived blocking handshake thread (Option A) running Schannel
AcceptSecurityContext, then decrypt on recv and encrypt on send; plaintext
connections replay the sniffed bytes straight into the existing parse path, so
the plaintext code path is otherwise unchanged. The identity is loaded once
from [tls] CertPath/KeyPath as PEM + CNG (PKCS#8) — the same key pair the Go
server uses — and a missing [tls] section, an empty value, or a load failure
leaves plaintext serving intact and fail-closes any TLS ClientHello.

Also fix two transport-reliability bugs that surfaced end-to-end:
- send path: encrypt only the freshly appended plaintext tail and post a
  single IOWrite wake-up guarded by a bSendPending flag, so back-to-back small
  sends (terminal resize -> COMMAND_NEXT -> resize) no longer run concurrent
  WSASends over the same mutable buffer (client saw DecryptMessage failures,
  with WSAGetLastError() = 6001 / ERROR_DECRYPTION_FAILED);
- client Recv: keep the handshake's unconsumed tail bytes in m_raw and drain
  them before the next recv, and log the real SECURITY_STATUS on
  DecryptMessage failure instead of the stale Winsock error.

Co-Authored-By: deepseek-v4-pro

docs/TransportTLS_Design.md

@@ -302,7 +302,7 @@ Set-ItemProperty -Path 'HKCU:\Software\<exehash>\settings' -Name 'tls_cert' -Val
 
 - **阶段 1（Go 服务端，本期优先）**：`Config` + `tls.Config` + `acceptLoop` 检测回放；明文路径零改动。可独立上线、独立验证——用 `openssl s_client` 或 Go 测试客户端即可联调 TLS 链路，无需真实客户端配合。
 - **阶段 2（Windows 客户端）**：证书检测 + TLS 栈 + fail-closed。
-- **阶段 3（MFC 服务端 `server/2015Remote`，延后）**：与 Go 服务端同款的 TLS 检测 + 解包接入。
+- **阶段 3（MFC 服务端 `server/2015Remote`，延后）**：与 Go 服务端同款的 TLS 检测 + 解包接入。设计稿见 `docs/TransportTLS_MFCServer_Design.md`。
 - **回滚**：服务端清空 `YAMA_TLS_CERT/KEY` 即回到纯明文；客户端删除注册表 `tls_cert` 值（或 `yama-ca.pem` 文件）即回明文。
 
 ## 12. 待定问题