Show figures of the routing table on the home page (closes #154)

The home page gets cards for the newest and oldest kept route, the newest
and oldest updated AS, the AS with the most prefixes, the ASes with a
single prefix and fifteen well-known networks. Two more cards count the
bogon announcements and the prefixes with several origin ASes. They link
to the new pages /bogons and /multiple-origins, which list up to 1,000
entries each.

The new package internal/bogon tells reserved prefixes and AS numbers,
from the IANA special-purpose and address space registries. Table.Overview
walks the table and lets the read lock go every 10,000 prefixes. The
server works the cards out once a minute and serves that copy. A new index
on asns(last_seen) finds the newest and oldest updated AS.

Model: opus-5-5

README.md

@@ -70,6 +70,44 @@ one of its peers sends an update.
 - `GET /` - Home page (HTML): headline figures, lookups by AS number and by IP
   address, and a search of AS names. A card shows the `curl` command that looks
   up the visitor's own address at `/ip/{addr}`, and that command's live answer.
+  Other cards show figures of the routing table, with links to the AS and prefix
+  pages:
+    - the newest and the oldest kept route, with its prefix, origin AS and time
+    - the AS last seen in an AS path most recently, and the one last seen
+      longest ago
+    - the AS that originates the most prefixes
+    - how many ASes originate a single prefix, and the most recently updated of
+      them
+    - the number of bogon announcements, linked to `/bogons`
+    - the number of prefixes with more than one origin AS, linked to
+      `/multiple-origins`
+    - well-known networks, such as Apple, Google, Cogent and MIT, each with its
+      number of prefixes and the time of its newest kept route, newest first
+
+    RouteWatch works these figures out once a minute, so they can be a minute
+    old.
+
+- `GET /bogons` - The bogon announcements (HTML): the routes whose prefix or
+  origin AS is reserved. A prefix is reserved when it lies inside one of these:
+    - a block that the IANA special-purpose address registries mark as not
+      globally reachable
+    - the multicast space, `224.0.0.0/4` and `ff00::/8`, which the IANA IPv4 and
+      IPv6 address space registries list as Multicast
+
+    A reserved AS number is one of these:
+    - an AS number for private use or for documentation
+    - `AS23456`
+    - the last AS number of the 16-bit or the 32-bit range
+
+    Address space and AS numbers that the IANA has not handed out yet are not
+    counted. Each row shows the prefix, the origin AS, the number of peers and
+    why it is a bogon. The page shows at most 1,000 rows, in prefix order, and
+    says how many there are in all.
+
+- `GET /multiple-origins` - The prefixes announced by more than one origin AS
+  (HTML). Each row shows the origin ASes and how many peers carry the prefix
+  with each. Like `/bogons`, it shows at most 1,000 rows, in prefix order, and
+  says how many there are in all.
 - `GET /status` - HTML status dashboard, which reads `/api/v1/stats`
 - `GET /status.json` - JSON statistics: feed, memory, database and route
   figures. The route figures come from the routing table. `oldest_route` and
@@ -248,6 +286,7 @@ routewatch/
 │   └── streamdumper/       # Debug utility for raw stream output
 ├── internal/
 │   ├── blocklist/          # Public IP blocklists: downloads and lookups
+│   ├── bogon/              # Reserved prefixes and AS numbers
 │   ├── routewatch/         # Core application logic
 │   ├── server/             # HTTP server and handlers
 │   ├── database/           # SQLite storage layer