Hygiene: scripts, CI, docs and manifests

- scripts/in-namespace.sh set CARGO_TARGET_DIR to the empty string when
  it was unset, which cargo refuses, and dropped RUSTFLAGS, so the run
  in the namespace rebuilt what kernel-tests.sh had just built. Each is
  passed on only where set.
- The README's list of unsupported features still had route reflection
  and graceful restart, both of which exist; the command-line reference
  lacked SIGUSR1 and exit status 75; the HTTP API reference did not say
  what errors look like.
- The event stream said "missed 0 events" for the best-routes stream,
  which does not count what it missed; it leaves the count out now.
- CI lints bgp-bench with its count-memory feature, which nothing built.
- bgp-bench had two unused dev-dependencies; bgp-integration-tests had
  three dependencies that only its tests and examples use.
- install.sh updates apt before installing; two-routers.sh loses an
  unused variable; small things in the web crate.

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

crates/bench/benches/full_table.rs

@@ -17,7 +17,7 @@ use std::sync::atomic::{AtomicUsize, Ordering};
 use std::time::{Duration, Instant};
 
 use bgp_bench::memory::{Region, describe};
-use bgp_bench::{Prefixes, Shape, announcements, encoded, wire_context};
+use bgp_bench::{Prefixes, Shape, announcements, encoded, env, wire_context};
 use bgp_integration_tests::test_peer::TestPeer;
 use bgp_integration_tests::test_server::TestServer;
 use bgp_packet::message::{Message, PathAttribute};
@@ -25,13 +25,6 @@ use bgp_primitives::ip_prefix::IpPrefix;
 use bgp_rib::table::{PeerIndex, PeerInfo, RibTable};
 use bytes::BytesMut;
 
-fn env(name: &str, default: usize) -> usize {
-    std::env::var(name)
-        .ok()
-        .and_then(|v| v.parse().ok())
-        .unwrap_or(default)
-}
-
 fn mib(bytes: usize) -> f64 {
     bytes as f64 / (1024.0 * 1024.0)
 }