[runyaga]

The HTTP integration tests called an external host, which now returns 503 and fails the integration job on main. CI should reach only package registries and GitHub.

• test/integration/loopback_http.dart starts an HTTP server on 127.0.0.1 with an ephemeral port. It answers any GET with a small JSON body, /large with a body over 64 KiB, and paths under /slow/ after 150 ms.
• The five test files that used the external host start the server in setUpAll and register its close at once, separately from the pool's. Every HttpClient connects directly, so a proxy in the environment cannot route loopback traffic.
• The four FFI SEGFAULT: use-after-free on sequential async host function calls in single session #271 tests use the /slow/ paths, because the crash they guard needs I/O that takes over 100 ms.
• Test counts are unchanged.

Local checks: dart analyze --fatal-infos test/integration clean, tool/dcm_host_gate.sh green, tool/test_cell.sh vm with the worker hook: integration 709 passed (floor 657), acceptance 12, portable 69.

Pre-PR review

Two review passes. The first asked for delayed responses in the #271 tests, cleanup that survives a failed pool start or close, and direct connections. All three are in the second commit. Reviewed at f0ec163.