[jonbaldie]

Closes #128

Summary

• Add a shared Varnish-container lookup for PURGE requests and VSL assertions.
• Add assert_no_vcl_errors <context> [pattern] and refactor the unsafe-location scenario to use it.
• Dump Varnish runtime VSL diagnostics alongside Compose logs when startup, readiness, or assertions fail, without changing the fixture exit status.

Verification

• Passed make test-compose-fixture-lifecycle, make test-purge, make test-hostile-unsafe-location, and make test-hostile-unsafe-location-canary.
• Temporarily removed the referenced-URI whitespace encoding; the unsafe-location test failed through assert_no_vcl_errors and showed the ban(): Expected && between conditions VSL record.
• The required local gate COMPOSE_PARALLEL_LIMIT=1 make -j1 test test-e2e-hard stopped at the existing test-smoke-runtime-interface target: its 5-second startup probe killed a docker run that took about 10 seconds to exit after printing the expected invalid-listen error. The gate did not reach the harness tests.