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/in-namespace.sh

@@ -11,8 +11,14 @@ sudo modprobe dummy 2>/dev/null || true
 sudo ip netns add "$namespace"
 trap 'sudo ip netns del "$namespace"' EXIT
 sudo ip netns exec "$namespace" ip link set lo up
+# Only the variables that are set are passed on: cargo refuses an empty
+# CARGO_TARGET_DIR, and the build settings have to be the same as the
+# prebuild's in scripts/kernel-tests.sh for it to be reused.
 sudo ip netns exec "$namespace" \
     setpriv --reuid "$(id -u)" --regid "$(id -g)" --init-groups \
     --inh-caps +net_admin --ambient-caps +net_admin \
-    env PATH="$PATH" HOME="$HOME" CARGO_TARGET_DIR="${CARGO_TARGET_DIR:-}" \
-    FERRIX_ROUTES="${FERRIX_ROUTES:-}" "$@"
+    env PATH="$PATH" HOME="$HOME" \
+    ${CARGO_TARGET_DIR:+CARGO_TARGET_DIR="$CARGO_TARGET_DIR"} \
+    ${RUSTFLAGS:+RUSTFLAGS="$RUSTFLAGS"} \
+    ${CARGO_TERM_COLOR:+CARGO_TERM_COLOR="$CARGO_TERM_COLOR"} \
+    ${FERRIX_ROUTES:+FERRIX_ROUTES="$FERRIX_ROUTES"} "$@"