Scope 16 / Phase 2: Extend Priority 7 routing — HighOutlierDensity KNN trigger and TailAsymmetry SkewSeverity upgrade

[DEVunderdog]

Parent

#105

What to build

Extend the Priority 7 distribution shape routing in _strategy_router.py with two new Scope 16 signals. Both apply only to non-BoundedDiscrete columns (BoundedDiscrete routing is handled inside its own Priority 3 gate).

TailAsymmetryTag severity upgrade

Before evaluating the Priority 7 distribution shape condition, compute an effective SkewSeverity:

• When NumericStats.tail_asymmetry_tag is TailAsymmetryTag.RightHeavy or TailAsymmetryTag.LeftHeavy, upgrade the effective SkewSeverity by one level (e.g. Moderate → High, High → Severe). Severe stays Severe.
• The stored NumericStats.skewness_severity is not modified — the upgrade is ephemeral, used only within this routing call.
• When tail_asymmetry_tag is Symmetric or None, effective severity equals stored severity.

NumericFlag.HighOutlierDensity as independent KNN trigger

Add NumericFlag.HighOutlierDensity as an independent escalation condition in the Priority 7 Minor/Moderate distribution shape gate, alongside KurtosisTag.Leptokurtic and SkewSeverity.Severe. A column at Minor or Moderate severity that has HighOutlierDensity set escalates to KNN (subject to size guards → Median fallback), regardless of its kurtosis tag or skewness.

Signal recording

When either signal triggers a routing change, append a diagnostic string to ColumnImputationRecord.signals:

• Tail asymmetry upgrade: "tail_asymmetry_right_heavy: effective_skew_severity upgraded from {stored} to {effective}" or the left_heavy variant
• Outlier density escalation: "high_outlier_density: escalated to KNN"

Acceptance criteria

• A column with SkewSeverity.Moderate and TailAsymmetryTag.RightHeavy routes to KNN (same as Severe) — ColumnImputationRecord.strategy == ImputationStrategy.KNN
• A column with SkewSeverity.Normal and TailAsymmetryTag.RightHeavy routes to KNN (upgraded from Normal to Moderate triggers distribution shape escalation path for Moderate+tail)
• NumericStats.skewness_severity is unchanged after routing — stored severity is not mutated
• A column with NumericFlag.HighOutlierDensity at Minor MCAR severity escalates to KNN, not Mean or Median
• A Mesokurtic column with HighOutlierDensity escalates to KNN (flag fires independently of kurtosis)
• ColumnImputationRecord.signals records the triggering signal for both mechanisms
• A column with TailAsymmetryTag.Symmetric and no HighOutlierDensity follows existing routing unchanged (regression test)
• All existing strategy router tests pass (no regressions)

Blocked by

• Blocked by Scope 16 / Phase 1: Data model — BimodalStats, TailAsymmetryTag, new NumericFlag members, NumericStats fields, NumericProfileConfig thresholds #229 (Phase 1 data model — TailAsymmetryTag, NumericFlag.HighOutlierDensity must exist in NumericStats)
• Blocked by Scope 16 / Phase 1: Compute tail asymmetry signal in NumericProfiler #231 (tail asymmetry computation — NumericStats.tail_asymmetry_tag must be populated by the profiler)
• Blocked by Scope 16 / Phase 1: Compute outlier density signal in NumericProfiler #232 (outlier density computation — NumericStats.outlier_density and NumericFlag.HighOutlierDensity must be populated by the profiler)