feat: convert serial console into the serial plugin

plugins/serial/README.md

@@ -0,0 +1,32 @@
+# Serial
+
+Serial console access over a device attached to the machine running Termix:
+the desktop app talks to it through the backend's `serialport` binding, and a
+browser without the desktop app falls back to the Web Serial API.
+
+Bundled with Termix and enabled by default.
+
+## Layout
+
+    manifest.json        id, capabilities and what this plugin contributes
+    src/backend/          activate(ctx) / deactivate(), the /console WS route
+    src/frontend/          the rail panel, the tab and their shared transport
+    locales/en.json       its strings (English only; the rest are translated)
+    tests/backend/         vitest, node
+    tests/frontend/        vitest, jsdom
+
+## Native dependency
+
+`serialport` (and its native `@serialport/bindings-cpp` binding) is this
+plugin's own npm dependency rather than a host-provided one, but the build
+never bundles it: see "native dependencies" in
+packages/plugin-sdk/ARCHITECTURE.md for why, and how Docker, Electron and dev
+each end up with a working binary anyway.
+
+## Commands
+
+    npm run build     bundle into dist/
+    npm run test      run this plugin's tests
+    npm run typecheck
+
+See packages/plugin-sdk/ARCHITECTURE.md for the contract this follows.