feat(electrobun): multi-machine relay + OTEL telemetry

Multi-machine relay:
- relay-client.ts: WebSocket client for agor-relay with token auth,
  exponential backoff (1s-30s), TCP probe, heartbeat (15s ping)
- machines-store.svelte.ts: remote machine state tracking
- RemoteMachinesSettings.svelte: machine list, add/connect/disconnect UI
- 7 RPC types (remote.connect/disconnect/list/send/status + events)

Telemetry:
- telemetry.ts: OTEL spans + OTLP/HTTP export to Tempo,
  controlled by AGOR_OTLP_ENDPOINT env var
- telemetry-bridge.ts: tel.info/warn/error frontend convenience API
- telemetry.log RPC for frontend→Bun tracing

ui-electrobun/src/mainview/machines-store.svelte.ts

@@ -0,0 +1,89 @@
+/**
+ * Svelte 5 rune store for remote machine state.
+ *
+ * Tracks connected machines, their status, and latency.
+ * Driven by remote.statusChange and remote.event messages from the Bun process.
+ */
+
+// ── Types ──────────────────────────────────────────────────────────────────
+
+export type MachineStatus = "connecting" | "connected" | "disconnected" | "error";
+
+export interface RemoteMachine {
+  machineId: string;
+  label: string;
+  url: string;
+  status: MachineStatus;
+  latencyMs: number | null;
+  error?: string;
+}
+
+// ── Store ──────────────────────────────────────────────────────────────────
+
+let machines = $state<RemoteMachine[]>([]);
+
+/** Add a machine to the tracked list. */
+export function addMachine(
+  machineId: string,
+  url: string,
+  label?: string,
+): void {
+  // Prevent duplicates
+  if (machines.some((m) => m.machineId === machineId)) return;
+
+  machines = [
+    ...machines,
+    {
+      machineId,
+      label: label ?? url,
+      url,
+      status: "connecting",
+      latencyMs: null,
+    },
+  ];
+}
+
+/** Remove a machine from tracking. */
+export function removeMachine(machineId: string): void {
+  machines = machines.filter((m) => m.machineId !== machineId);
+}
+
+/** Update the status of a tracked machine. */
+export function updateMachineStatus(
+  machineId: string,
+  status: MachineStatus,
+  error?: string,
+): void {
+  machines = machines.map((m) =>
+    m.machineId === machineId
+      ? { ...m, status, error: error ?? undefined }
+      : m,
+  );
+}
+
+/** Update the measured latency for a machine. */
+export function updateMachineLatency(
+  machineId: string,
+  latencyMs: number,
+): void {
+  machines = machines.map((m) =>
+    m.machineId === machineId ? { ...m, latencyMs } : m,
+  );
+}
+
+/** Get all tracked machines (reactive). */
+export function getMachines(): RemoteMachine[] {
+  return machines;
+}
+
+/** Get a single machine by ID (reactive). */
+export function getMachineStatus(
+  machineId: string,
+): RemoteMachine | undefined {
+  return machines.find((m) => m.machineId === machineId);
+}
+
+/** Get count of connected machines (reactive). */
+export function getConnectedCount(): number {
+  return machines.filter((m) => m.status === "connected").length;
+}