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/passive/packet_capture.py

@@ -23,7 +23,7 @@ from pathlib import Path
 from typing import Optional
 
 from modules.base import BaseModule
-from utils.networking import get_primary_interface
+from utils.networking import detect_interface_with_retry
 
 logger = logging.getLogger("sensor.passive.packet_capture")
 
@@ -64,7 +64,12 @@ class PacketCapture(BaseModule):
         if self._running:
             return
 
-        iface = self.config.get("interface") or get_primary_interface() or self.DEFAULT_INTERFACE
+        iface = self.config.get("interface") or detect_interface_with_retry(
+            max_retries=3,
+            retry_delay=5,
+            exponential=True,
+            config_interface=self.DEFAULT_INTERFACE
+        )
         snap_len = self.config.get("snap_length", self.DEFAULT_SNAP_LEN)
         rotation = self.config.get("rotation_minutes", 60) * 60
         if rotation <= 0: