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>

.github/workflows/ci.yml

@@ -24,6 +24,10 @@ jobs:
         run: cargo fmt --all -- --check
       - name: Lint
         run: cargo clippy --locked --workspace --all-targets -- -D warnings
+      # The memory-counting allocator of the benchmarks is behind a feature
+      # that the workspace build does not turn on.
+      - name: Lint the benchmarks with memory counting
+        run: cargo clippy --locked -p bgp-bench --all-targets --features count-memory -- -D warnings
       - name: Test
         run: cargo test --locked --workspace
       # A link in the documentation to something that is not there.