Floor perf-test timing denominator to deflake CI (#1073)

test_generate_does_not_scale_linearly flaked twice this week (5.3x on
windows-3.10, 5.9x on macos-3.10) with both absolute times under a quarter
second, where scheduler noise dominates the ratio. Floor the denominator at
100ms; a real pathological regression still trips the assertion.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: PavelMakarchuk

changelog.d/fix-flaky-generate-perf-test.fixed.md

@@ -0,0 +1 @@
+Floor the generate-phase performance test's timing denominator at 100ms so scheduler noise on fast CI runners no longer flakes the 5x scaling assertion.

tests/test_performance.py

@@ -183,7 +183,13 @@ def test_generate_does_not_scale_linearly(self):
             times[n] = time.time() - t0
             dataset.cleanup()
 
-        ratio = times[500] / max(times[100], 0.01)
+        # Floor the denominator at 100ms: when the 100-record base runs in a
+        # few tens of milliseconds, scheduler noise dominates the ratio and
+        # the test flakes on CI (observed 5.3x/5.9x on runs where both
+        # absolute times were well under a quarter second). A genuinely
+        # pathological (e.g. quadratic row-loop) regression still trips the
+        # assertion because times[500] grows into whole seconds.
+        ratio = times[500] / max(times[100], 0.1)
         assert ratio < 5.0, (
             f"Generate phase scaled {ratio:.1f}x for 5x more records "
             f"(100: {times[100]:.2f}s, 500: {times[500]:.2f}s). "