feat(lem): integrate Poindexter for spatial score indexing and analytics
- Add feature vector extraction (6D grammar, 8D heuristic, 14D combined)
- Add KDTree ScoreIndex with cosine distance for probe clustering
- Add score distribution analytics (percentiles, variance, skewness)
- Add grammar-profile dedup filtering to distill pipeline
- Add spatial gap detection (FindGaps) for coverage analysis
- Wire analytics into coverage CLI (PrintScoreAnalytics)
New files: features.go, cluster.go, analytics.go + tests
Modified: distill.go (dedup filter), coverage.go (analytics output)
Dep: github.com/Snider/Poindexter
Co-Authored-By: Virgil <virgil@lethean.io>
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