Robust interface detection — retry fallback chain for headless boot (#206)

Fixes interface auto-detection failures on Pi Zero 2W where wlan0 doesn't exist
at boot. Implementation:

- Added detect_interface_with_retry() function with 3x retry fallback chain
- Config-supplied interface preferred, then /proc/net/route, then UP interface
- Exponential backoff: 5s, 10s, 20s delays between retries
- Filters excluded prefixes: lo, tailscale*, wg*, tun*, docker*, veth*, br-*
- Used in Engine.__init__(), PacketCapture.start(), MacManager.start()
- Full test coverage: 10 tests for detection, backoff, filtering

Addresses issue where network stack initializes after boot and interfaces
become available after 5-10 second delay.

modules/stealth/mac_manager.py

@@ -20,7 +20,7 @@ from typing import Optional
 from modules.base import BaseModule
 from utils.networking import (
     get_mac,
-    get_primary_interface,
+    detect_interface_with_retry,
     get_wifi_interfaces,
     set_mac,
 )
@@ -74,7 +74,12 @@ class MacManager(BaseModule):
             "primary_interface", "auto"
         )
         if self._eth_iface == "auto":
-            self._eth_iface = get_primary_interface()
+            self._eth_iface = detect_interface_with_retry(
+                max_retries=3,
+                retry_delay=5,
+                exponential=True,
+                config_interface=None
+            )
 
         wifi_ifaces = get_wifi_interfaces()
         wifi_cfg = self.config.get("network", {}).get("wifi", {}).get("interface", "wlan0")