Look up IP addresses by prefix at each mask length (closes #48)

Looking up an address read every IPv6 live route and, for IPv4, about
half of the range index. Both families now look the address's prefix up
at each mask length, longest first, on the prefix index: at most 33 or
129 indexed lookups, however many routes are live.

The feed sends IPv6 withdrawals uncompressed but announcements
compressed, so the prefix handler now stores every prefix in the text
form net/netip prints, the form the lookup builds. IPv6 withdrawals now
remove their routes.

The IPv4 range columns, their index and the code that fills them are
removed, and so are the unused GetASInfoForIP lookups.

Model: opus-5-5

TODO.md

@@ -28,6 +28,11 @@ The other open issue is https://git.eeqj.de/sneak/routewatch/issues/30.
 
 # Completed Steps
 
+- 2026-10-03: looking up an IP address no longer reads every IPv6 route:
+  both families find the most specific live route with at most 33 or 129
+  lookups on the prefix index, and the IPv4 range columns are gone. Prefixes
+  from the feed are stored in one text form, so an IPv6 withdrawal, which the
+  feed sends uncompressed, now removes its route (closes #48)
 - 2026-10-02: a plain `docker build .` stamps the commit's tag or short
   commit (`git describe --tags --always`) into the page footer instead of
   `unknown`: `.dockerignore` sends `.git` without `.git/config`, a `VERSION`