Add code-review canary fixture (push A)

Two planted correctness bugs for the P3 live acceptance cycles on this
draft PR.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W5V4zmmAs92ngEsSKrvpAJ

tools/code-review-canary/canary.py

@@ -0,0 +1,27 @@
+"""Deliberately flawed fixture for the P3 incremental live acceptance.
+
+Planted correctness bugs, so a reviewed push with post_pr enabled is
+guaranteed inline-postable findings:
+
+- ``percentile`` indexes past the end of the list when fraction is 1.0.
+- ``moving_average`` divides every window by the full window size, so
+  the tail averages are too small.
+
+This file exists only on the calibration draft PR and is never merged.
+"""
+
+
+def percentile(values, fraction):
+    """Return the value at the given fraction of the sorted input."""
+    ordered = sorted(values)
+    index = int(len(ordered) * fraction)
+    return ordered[index]
+
+
+def moving_average(values, window):
+    """Average each window of the input, including the shorter tail."""
+    averages = []
+    for start in range(len(values)):
+        chunk = values[start:start + window]
+        averages.append(sum(chunk) / window)
+    return averages