Add Phase 0a black-box integration test suite

81 tests across 11 files plus a 5-module harness (load, pg, streams,
containers, payloads), covering all eleven Phase 0 goals and all 13 named
chaos-flag pathologies. Talks to the system only over HTTP, Postgres, Redis,
and the Docker CLI -- no imports of application code. RED-BASELINE.md records
the suite failing red against the empty repo before any implementation
existed; README.md documents the request-contract fields invented to fill
gaps in the spec, measurement methodology, and known-flaky tests.

This suite is a frozen contract from this point forward per CLAUDE.md.

pytest.ini

@@ -0,0 +1,12 @@
+[pytest]
+addopts = --strict-markers
+asyncio_mode = auto
+timeout = 300
+testpaths = tests
+markers =
+    contract: fast functional behaviour under chaos-clean (< 60s total)
+    seed: seed data integrity checks (< 20s total)
+    baseline: the clean system is genuinely healthy (< 90s total)
+    pathology: one differential test per pathology (5-12 min each)
+    slow: additional marker on the soak-ish pathology tests (unbounded_cache, cold_cache)
+    guard: "no observability yet" assertions -- delete at Phase 2 (< 5s total)