chore: dev-stack switches, shared e2e harness, and desktop shell rules

- `LIVE_RELOAD=false npm run dev` keeps the renderer alive across a machine
  suspend; the reload client otherwise destroys the session under test.
- `dev-peer.sh` plus a separate userdata dir runs a second local peer.
- `tools/voice-probe.js` samples peer state and RTP counters from a live
  window, persisting to localStorage so a renderer reload cannot erase it.
- e2e helpers for voice pairs, peer-role election, and a TURN relay.
- Electron single-instance and dev-client-load decisions move into rules
  files with colocated specs.

electron/app/second-instance.rules.ts

@@ -0,0 +1,23 @@
+export const DEV_RELOAD_EXISTING_ARG = '--metoyou-dev-reload-existing';
+
+export type SecondInstanceAction = 'reload-existing' | 'focus';
+
+export interface SecondInstanceInput {
+  argv: string[];
+  devSingleInstanceExitCode: number | null;
+}
+
+/**
+ * A dev launch always carries `--metoyou-dev-reload-existing`, so the running
+ * instance reloads in place. It must never answer by relaunching itself: the
+ * successor inherits the same argument and asks for the single-instance lock
+ * while the dying parent still holds it, so the refused successor fires
+ * `second-instance` again and the pair respawns forever.
+ */
+export function resolveSecondInstanceAction(input: SecondInstanceInput): SecondInstanceAction {
+  const isDevelopmentLaunch = input.devSingleInstanceExitCode != null;
+
+  return isDevelopmentLaunch && input.argv.includes(DEV_RELOAD_EXISTING_ARG)
+    ? 'reload-existing'
+    : 'focus';
+}