add steric maps pipeline & update analysis scripts

Add buried volume & buried shell computation scripts for capped BRICS
fragments with phenyl and t-butyl caps. Includes multi-radius scanning,
Boltzmann-weighted summaries, PCA analysis, and cap comparison parity
plots. Clean up r_group_fragmentation (remove intermediate files moved
to steric_maps pipeline). Minor analysis script cleanups.

Author: bobbypaton

.gitignore

@@ -33,3 +33,9 @@ htmlcov/
 .claude/
 ct4c00784_si_001.pdf
 TODO
+analysis/r_group_fragmentation/250k_rndm_zinc_drugs_clean_3_brics_counts.csv
+
+# Large data files
+analysis/steric_maps/*.sdf
+analysis/steric_maps/*_buried_shell.csv
+analysis/steric_maps/*_buried_shell_summary.csv