fix(tuning): a boosted laggard never runs through zero

Humpty Dump replayed on test-mcp d02d82c (2026-10-20): the insight
repair_unit_utilization_drag promised 0.18x. _boost_weak_insights scales the
distance from 1.0 linearly, so two rounds took the magnitude to -1.1x and the
report said "needed <= -0.05x to pass". A multiplier below zero is a sign
error, not a weaker segment. Boosted laggards now floor at LAGGARD_FLOOR
(0.05x of baseline).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

demoprep_app/pipeline/build_demo.py

@@ -44,6 +44,8 @@ MAX_TUNING_ROUNDS = 3
 # When an insight is too weak, multiply its distance-from-1.0 by this.
 BOOST_FACTOR = 1.6
 # A BOUNDED measure (a rate, a score, a share) is pushed in its band instead:
+# A boosted laggard never goes below this multiple of its baseline.
+LAGGARD_FLOOR = 0.05
 # PlantedInsight.drive_boost grows by BOOST_FACTOR per round up to this.
 MAX_DRIVE_BOOST = 4.0
 # The share of a band's theoretical delivery (position 0.5 -> 0.5 +/-
@@ -506,7 +508,12 @@ def _boost_weak_insights(blueprint: DemoBlueprint, report: ValidationReport) ->
             deliverable = _deliverable_band_ratio(blueprint, insight)
             if deliverable is None:
                 distance = insight.magnitude - 1.0
-                insight.magnitude = round(1.0 + distance * BOOST_FACTOR, 3)
+                # A laggard's distance is negative, and scaling it linearly
+                # runs through zero: Humpty Dump on test-mcp d02d82c (2026-10-20)
+                # promised 0.18x, two boosts made it -1.1x, and the report read
+                # "needed <= -0.05x". A multiplier below LAGGARD_FLOOR is not a
+                # weaker segment, it is a sign error.
+                insight.magnitude = max(round(1.0 + distance * BOOST_FACTOR, 3), LAGGARD_FLOOR)
             else:
                 floor, cap = deliverable
                 laggard = insight.magnitude < 1.0

tests/test_build_abort_policy.py

@@ -1106,6 +1106,21 @@ def test_the_tuning_boost_does_not_raise_the_pass_bar():
     assert laggard.direction == "below baseline"
 
 
+def test_a_laggard_boost_never_runs_through_zero():
+    """Humpty Dump on test-mcp d02d82c (2026-10-20): a 0.18x laggard boosted
+    twice became -1.1x and the report said "needed <= -0.05x"."""
+    import types
+
+    bp = _bp()
+    insight = bp.insights[0]
+    insight.magnitude = 0.18
+    report = types.SimpleNamespace(failed_insights=lambda: [types.SimpleNamespace(insight_id=insight.id)])
+    for _ in range(3):
+        build_demo_module._boost_weak_insights(bp, report)
+    assert insight.authored_magnitude == 0.18
+    assert build_demo_module.LAGGARD_FLOOR <= insight.magnitude < 0.18
+
+
 def test_a_missing_account_is_a_plain_instruction_naming_user_and_cluster():
     from thoughtspot_errors import trusted_auth_failure_message