Add internal/routetable, the routing table in memory (part of #122)

A new package holds every prefix some peer carries. For each prefix and
origin AS it keeps the set of peers that carry it, one bit per peer,
and the two newest of their routes in full. It applies announcements,
withdrawals and peers' sessions going down by the rule confirmed on the
issue, and records which pairs changed for the database writes to come.
It answers the IP lookup, an AS's prefixes, a prefix's origins, a sample
of one mask length, and the counts the stats show. Nothing uses it yet.

The peer sets use github.com/bits-and-blooms/bitset. The kept routes'
times are held in order in github.com/google/btree, so the oldest and
newest need no scan.

Model: opus-5-5

TODO.md

@@ -33,6 +33,10 @@ tested.
 
 # Completed Steps
 
+- 2026-10-27: a new package, `internal/routetable`, holds the whole routing
+  table in memory. For each prefix and origin AS it keeps the peers that carry
+  it, one bit per peer, and the two newest of their routes. Nothing uses it yet
+  (part of #122)
 - 2026-10-27: each result of the IP lookup lists the public blocklists the
   address is on, in `blocklists`. The lists are DShield, Feodo Tracker, two
   FireHOL lists, Spamhaus DROP and the Tor exit list. RouteWatch downloads each
@@ -244,4 +248,6 @@ tested.
 
 # Future Steps
 
-None open.
+- the rest of https://git.eeqj.de/sneak/routewatch/issues/122, one PR each: the
+  routing table in the database, the prefix handler writing to it, the pages and
+  the API reading from it, then removing the per-peer route tables