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/integration-tests/Cargo.toml

@@ -17,11 +17,14 @@ bgp-rpki.workspace = true
 bgp-server.workspace = true
 bgp-web.workspace = true
 bytes.workspace = true
-clap.workspace = true
 eyre.workspace = true
+tokio.workspace = true
+
+# Used by the examples and the tests only.
+[dev-dependencies]
+clap.workspace = true
 reqwest.workspace = true
 serde_json.workspace = true
-tokio.workspace = true
 
 [lints]
 workspace = true