Ignore routes shorter than /8 or /16 in the IP lookup (closes #57)

The IP lookup tried every mask length down to /0, so a default route or
a very short route such as 2000::/3 was reported as the owner of any
address it covered until a more specific route was seen. Such a route
only says where a peer sends traffic it has no better route for.

The lookup now stops at /8 for IPv4 and /16 for IPv6; an address covered
only by shorter routes gets "no route found". The live tables still
store those routes. README states the limit.

Model: opus-5-5

README.md

@@ -144,8 +144,9 @@ routewatch/
 - **Reconnection**: Exponential backoff (5s-320s) with reset after 30s of stable connection
 - **IP Lookup**: the most specific live route is found by looking up the
   address's prefix at each mask length, longest first, in the prefix index
-  (at most 33 lookups for IPv4, 129 for IPv6); prefixes are stored in the
-  text form Go's `net/netip` prints
+  (at most 25 lookups for IPv4, 113 for IPv6); prefixes are stored in the
+  text form Go's `net/netip` prints. Routes shorter than /8 for IPv4 or /16
+  for IPv6, such as a default route, are not used.
 
 ### Database Schema