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.

tests/harness/streams.py

@@ -0,0 +1,28 @@
+"""Redis XINFO GROUPS helpers, per phase-0a-test-suite-spec.md §7."""
+import time
+
+
+def group_lag(redis_client, stream: str, group: str) -> dict:
+    """Return the XINFO GROUPS entry for `group` on `stream` as a dict."""
+    groups = redis_client.xinfo_groups(stream)
+    for g in groups:
+        if g.get("name") == group:
+            return g
+    raise ValueError(f"consumer group {group!r} not found on stream {stream!r}: {groups}")
+
+
+def wait_for_drain(redis_client, stream: str, group: str, timeout: float = 15.0) -> None:
+    """Block until `group`'s lag and pending count both reach 0, or raise TimeoutError."""
+    deadline = time.monotonic() + timeout
+    last = None
+    while time.monotonic() < deadline:
+        g = group_lag(redis_client, stream, group)
+        lag = g.get("lag") or 0
+        pending = g.get("pending") or 0
+        last = (lag, pending)
+        if lag == 0 and pending == 0:
+            return
+        time.sleep(0.25)
+    raise TimeoutError(
+        f"group {group!r} on {stream!r} did not drain within {timeout}s (last lag/pending={last})"
+    )