接续L4-TOOL-DB浏览器Provider协议

结论：浏览器 Worker 已支持可插拔 Python runtime provider 的 line I/O，缺真实 Python/WASM runtime 时仍保持阻塞不解锁。

wasm-port/runtime/sdk/README.md

@@ -577,7 +577,9 @@ private module paths:
   boundary. The bundled worker reports
   `blocked_browser_python_wasm_runtime_missing` until a real Python/WASM runtime
   is installed; the Worker shell alone must not mark `DB_PROGRAM` execution
-  ready.
+  ready. The Worker supports a provider protocol (`start`, `writeLine`,
+  `readLine`, `close`) so a future Python/WASM runtime can provide line-based
+  `DB_PROGRAM` I/O without changing `ToolDbProcessPort`.
 - `createLinuxCncToolDbNodeRuntimeAdapter()` in
   `runtime/sdk/src/tool-db-node-runtime-adapter.js` for Node/native verification
   of the real LinuxCNC `DB_PROGRAM` child process. This adapter is intentionally