feat: expose dongle debug info through the C API

Add an opaque xone_dongle session handle to the C ABI bridge:
xone_open() probes, resets, and loads the firmware (path optional),
and xone_pid/xone_chip_id/xone_mac_address/xone_firmware_build return
debug info for the open session. chip captures the firmware build
string from the image header during load.

The Swift app opens a session when the dongle appears and shows PID,
chip ID, MAC address, and firmware build string in a debug section.

Co-Authored-By: qwen3.8-27b@q2_k_xl: added C API session handle and debug UI

src/app/main.swift

@@ -26,6 +26,7 @@ final class AppDelegate: NSObject, NSApplicationDelegate {
 
 struct ContentView: View {
     @State private var donglePresent = false
+    @State private var session: OpaquePointer? = nil
 
     private let timer = Timer.publish(every: 1.0, on: .main, in: .common)
         .autoconnect()
@@ -42,6 +43,10 @@ struct ContentView: View {
 
             statusView
 
+            if let session {
+                debugView(session)
+            }
+
             Divider()
 
             controllersView
@@ -62,10 +67,43 @@ struct ContentView: View {
         .frame(width: 420)
         .frame(minHeight: 320)
         .onReceive(timer) { _ in
-            donglePresent = xone_dongle_present()
+            let present = xone_dongle_present()
+            if present && session == nil {
+                session = xone_open("firmware/xow_dongle.bin")
+            } else if !present, let s = session {
+                xone_close(s)
+                session = nil
+            }
+            donglePresent = present
         }
     }
 
+    private func debugView(_ session: OpaquePointer) -> some View {
+        VStack(alignment: .leading, spacing: 4) {
+            Text("Debug")
+                .font(.headline)
+            debugRow("PID", String(format: "0x%04X", xone_pid(session)))
+            debugRow("Chip ID", String(format: "0x%04X", xone_chip_id(session)))
+            debugRow("MAC", String(cString: xone_mac_address(session)))
+            debugRow("Firmware", firmwareBuildText(session))
+        }
+    }
+
+    private func debugRow(_ label: String, _ value: String) -> some View {
+        HStack {
+            Text(label)
+                .foregroundStyle(.secondary)
+            Spacer()
+            Text(value)
+                .font(.system(.body, design: .monospaced))
+        }
+    }
+
+    private func firmwareBuildText(_ session: OpaquePointer) -> String {
+        let build = String(cString: xone_firmware_build(session))
+        return build.isEmpty ? "not loaded" : build
+    }
+
     private var statusView: some View {
         HStack(spacing: 8) {
             Circle()