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>

scripts/interop/install.sh

@@ -3,6 +3,7 @@
 # BIRD from the distribution, Routinator from crates.io (a few minutes to
 # build), and radvd, which tells FRR who is on a link.
 set -eu
+sudo apt-get update
 sudo DEBIAN_FRONTEND=noninteractive apt-get install -y frr bird2 jq curl radvd
 # They are run by the lab, in namespaces, not as services of this machine.
 sudo systemctl disable --now frr bird radvd 2>/dev/null || true