[NOTICKET] fix(tools): analyze_ab_test quality — KeyError on the too-few-rows path

The quality block (added in 2082eba1) read test['p_value'] directly, but _welch_test's too-few-rows branch returns no 'p_value' key -> KeyError, so analyze_ab_test crashed on a valid 'too few rows for a t-test' case (became an error envelope via the never-throw seam instead of a graceful result). Fix: test.get('p_value'). The too-few case now returns significant=None + quality {value: None, weak: True}. Caught by a local ab_test test.

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

src/tools/analytics/ab_test.py

@@ -207,7 +207,7 @@ def analyze_ab_test(
         # zero variance) — also weak, for the same reason. CK7 surfaces it.
         "quality": {
             "metric": "p_value",
-            "value": None if test["p_value"] is None else round(test["p_value"], 4),
+            "value": None if test.get("p_value") is None else round(test["p_value"], 4),
             "floor": alpha,
             "weak": not test["significant"],
             "note": (