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.

core/engine.py

@@ -24,7 +24,7 @@ from typing import Optional, Type
 from core.bus import EventBus, Event
 from core.state import StateManager
 from modules.base import BaseModule
-from utils.networking import get_primary_interface
+from utils.networking import detect_interface_with_retry
 
 logger = logging.getLogger("sensor.engine")
 
@@ -240,12 +240,17 @@ class Engine:
         self._scope = config.get("scope", {})
         self._phase = config.get("engagement_phase", {}).get("mode", "passive_only")
 
-        # Resolve "auto" interface to actual interface name
+        # Resolve "auto" interface to actual interface name with retry fallback
         if self.config.get("network", {}).get("primary_interface") == "auto":
-            actual_iface = get_primary_interface()
+            actual_iface = detect_interface_with_retry(
+                max_retries=3,
+                retry_delay=5,
+                exponential=True,
+                config_interface=None
+            )
             if actual_iface:
                 self.config.setdefault("network", {})["primary_interface"] = actual_iface
-                logger.debug("Resolved auto interface to: %s", actual_iface)
+                logger.info("Resolved auto interface to: %s (with retry)", actual_iface)
         self.capture_bus = None
         self._lock = multiprocessing.Lock()
         logger.info("Engine initialized (tier=%s, phase=%s)", self._tier, self._phase)