feat: 浏览器端 CH340/CH341 WebUSB 驱动 + navigator.serial 垫片

- ch340-webusb.js: WebUSB 上完整的 USB-Serial 栈
  · CH340/CH341 vendor 控制传输初始化（对齐 Linux ch341.c）
  · 波特率分频算法 calcCh34xBaud（1532620800 基准）
  · bulk IN 32 深读队列 + ReadableStream / 4096 分片写 + WritableStream
  · DTR/RTS（0xA4 取反写入）、拔插事件、诊断计数
  · 附带 CDC-ACM(CH343/CH9102/CH347)、CP210x、FTDI(含 modem 字节剥离)、PL2303(HXN/HX/01/T)
- ch340-serial-shim.js: 安卓上补出 navigator.serial，老代码零改动可用
- ch340-test.html: 连接/收发/HEX/DTR-RTS/回环自测 测试台
- docs/ch34x-registers.md: 寄存器与控制传输逆向说明
- test/driver.test.mjs: 15 项 mock 断言（node，无需硬件）

LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2026 jiangchen
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.