session-memory: Phase 4 Measure — baseline, effectiveness, trend (WP-0009)

Closes the loop. metrics.py: fleet metrics (infra-overhead share, error rate,
schema-thrash, token percentiles, success) + persisted baseline trend. effect.py:
before/after per-pattern effectiveness with an improved verdict per metric.
measure entrypoint with trend + --since effectiveness + JSON. Recorded pre-fix
baseline: 27 sessions, overhead median 11.7%, error rate 0.96, schema-thrash 8.
13 new tests; suite 139/139. Capture->Detect->Curate->Distribute->Measure complete.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_measure_effect.py

@@ -0,0 +1,49 @@
+"""Before/after effectiveness tests (WP-0009 T02)."""
+
+import os
+import sys
+
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+from session_memory.measure.effect import effectiveness, split_by_date  # noqa: E402
+
+
+def _digest(ts, tools=None, errors=0, outcome="success"):
+    return {
+        "started_at": ts, "outcome": outcome,
+        "cost": {"input_tokens": 100, "output_tokens": 0},
+        "tool_histogram": tools or {"Bash": 10},
+        "error_snippets": [{"fingerprint": f"e{i}", "count": 1} for i in range(errors)],
+    }
+
+
+def test_split_by_date():
+    digs = [_digest("2026-06-01"), _digest("2026-06-05"), _digest("2026-06-10")]
+    before, after = split_by_date(digs, "2026-06-05")
+    assert len(before) == 1 and len(after) == 2  # >= applied_at goes to after
+
+
+def test_effectiveness_detects_improvement():
+    # before: lots of errors + hub overhead; after: clean
+    before = [_digest("2026-06-01", tools={"mcp__state-hub__x": 8, "Bash": 2}, errors=3)
+              for _ in range(3)]
+    after = [_digest("2026-06-10", tools={"Bash": 10}, errors=0) for _ in range(3)]
+    e = effectiveness(before + after, "2026-06-05", label="read-before-edit")
+    assert not e["insufficient_data"]
+    assert e["n_before"] == 3 and e["n_after"] == 3
+    assert e["deltas"]["error_rate"]["improved"] is True
+    assert e["deltas"]["infra_overhead_share_median"]["improved"] is True
+    assert e["deltas"]["error_rate"]["change"] < 0
+
+
+def test_effectiveness_insufficient_data():
+    e = effectiveness([_digest("2026-06-01")], "2026-06-05")
+    assert e["insufficient_data"] is True
+    assert e["deltas"] == {}
+
+
+def test_success_rate_higher_is_better():
+    before = [_digest("2026-06-01", outcome="fail") for _ in range(2)]
+    after = [_digest("2026-06-10", outcome="success") for _ in range(2)]
+    e = effectiveness(before + after, "2026-06-05")
+    assert e["deltas"]["success_rate"]["improved"] is True