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/pg.py

@@ -0,0 +1,74 @@
+"""pg_stat_statements deltas and EXPLAIN helpers, per phase-0a-test-suite-spec.md §7."""
+from contextlib import contextmanager
+
+import psycopg
+
+
+class _Counter:
+    """Populated with `.calls` (and `.total_time_ms`) once the `with` block exits."""
+
+    def __init__(self):
+        self.calls = None
+        self.total_time_ms = None
+
+    def __repr__(self):
+        return f"Counter(calls={self.calls}, total_time_ms={self.total_time_ms})"
+
+    def __str__(self):
+        return repr(self)
+
+
+@contextmanager
+def statement_count(dsn):
+    """Reset pg_stat_statements, run the caller's block, yield the call-count delta.
+
+    pg_stat_statements is reset globally, so this is only meaningful when
+    nothing else is hitting the database concurrently during the `with` block
+    -- true for the single-request pathology assertions this is built for.
+    """
+    counter = _Counter()
+    with psycopg.connect(dsn, autocommit=True) as conn:
+        with conn.cursor() as cur:
+            cur.execute("SELECT pg_stat_statements_reset()")
+        try:
+            yield counter
+        finally:
+            with conn.cursor() as cur:
+                cur.execute(
+                    "SELECT COALESCE(SUM(calls), 0), COALESCE(SUM(total_exec_time), 0) "
+                    "FROM pg_stat_statements"
+                )
+                calls, total_time_ms = cur.fetchone()
+            counter.calls = calls
+            counter.total_time_ms = total_time_ms
+
+
+def explain(dsn, sql, params=None):
+    """Run EXPLAIN (ANALYZE, FORMAT JSON) and return the plan dict."""
+    with psycopg.connect(dsn, autocommit=True) as conn:
+        with conn.cursor() as cur:
+            cur.execute(f"EXPLAIN (ANALYZE, FORMAT JSON) {sql}", params or ())
+            (plan,) = cur.fetchone()
+            return plan[0]
+
+
+def plan_node_types(plan: dict) -> set:
+    """Flatten an EXPLAIN JSON plan (or its top-level {"Plan": ...} wrapper)
+    into the set of "Node Type" values it contains, at any depth."""
+    types: set = set()
+    root = plan.get("Plan", plan) if isinstance(plan, dict) else plan
+
+    def walk(node):
+        if isinstance(node, dict):
+            if "Node Type" in node:
+                types.add(node["Node Type"])
+            for key, value in node.items():
+                if key == "Plans" and isinstance(value, list):
+                    for child in value:
+                        walk(child)
+        elif isinstance(node, list):
+            for item in node:
+                walk(item)
+
+    walk(root)
+    return types