Code health after review: docs, metrics, structure, one more fuzz target

What the code-health review found, and what a fuzz target written for it
found in its first seconds.

- A set of attributes with the same attribute twice is refused by the
  routing table. Its readers (the selection key, the path, the next hop)
  could each take a different one of the two. The decoder never lets one
  through; the new fuzz target `attrs` found that the table would.
- bgp_peer_up{peer}, to alert on a peer going away, and bgp_build_info.
  Every metric is there from the first scrape. Alerts for a peer down,
  hijack attempts and kernel errors are in the README.
- `bgp --check config` validates a configuration without starting, and
  `--version` says which build it is. CI runs the first on the example
  configuration, which a test now also parses with its commented fields
  put in; the example had to learn about the next hop check.
- Path attributes have a module of their own, packet::attributes,
  re-exported from packet::message as before.
- policy::From, which shadowed the trait, is RouteSource. The server's
  connection, dialer and fsm modules are private. Logging arguments live
  in the binary; the server library no longer depends on clap.
- One is_unicast_link_local (the standard library's) instead of two of
  our own; SelectionKey::origin(); one place where an Import is made.
- READMEs say what the daemon does today rather than what it did not do
  a year ago. Documentation links are checked in CI. Unused dependencies
  and bgp_web::serve are gone; the Docker build is --locked and does not
  ship the fuzz corpus.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Dockerfile

@@ -3,7 +3,7 @@
 FROM rust:1.92-bookworm AS build
 WORKDIR /src
 COPY . .
-RUN cargo build --release -p bgp
+RUN cargo build --release --locked -p bgp
 
 FROM debian:bookworm-slim
 COPY --from=build /src/target/release/bgp /usr/local/bin/bgp