refactor(router): centralize bridge fan-out and type drop reasons

Replace the eight hand-rolled companion-bridge delivery loops in the
packet router with a single _fan_out_to_bridges helper that reads
HandlerResult.authenticated directly, so a broken handler contract
surfaces instead of being hidden by getattr hedges.

Introduce a DropReason str-Enum in the engine for the seventeen fixed
drop reasons (with __str__ returning the value, since str() of a
str-Enum changed on Python 3.11+) and derive the router's
expected-drop check from it, retiring the string-prefix tuple.
Detailed variants keep their suffixes embedded in the reason string.

Split the companion-delivery dedupe cache into a pure check and an
explicit mark so PATH and protocol responses record delivery only
after the fan-out runs — a copy where every bridge raised is retried
on the next copy instead of being suppressed for the full TTL.

repeater/handler_helpers/advert.py

@@ -15,6 +15,8 @@ from typing import Dict, Optional, Tuple
 
 from openhop_core.node.handlers.advert import AdvertHandler
 
+from repeater.engine import DropReason
+
 logger = logging.getLogger("AdvertHelper")
 
 
@@ -551,7 +553,7 @@ class AdvertHelper:
             if not advert_data or not advert_data.get("valid"):
                 logger.warning("Invalid advert packet received, dropping.")
                 packet.mark_do_not_retransmit()
-                packet.drop_reason = "Invalid advert packet"
+                packet.drop_reason = DropReason.INVALID_ADVERT
                 return
 
             # Extract data from parsed advert