fix: rescue gnomAD genes with non-Ensembl IDs via gene_symbol fallback

gnomAD v4.1 gene_id column is mixed: ~101K Ensembl IDs + ~111K NCBI numeric.
Now falls back to gene_symbol → gene_universe lookup for non-ENSG entries.

Coverage: 78.5% → 90.3% (17,875 → 20,555 genes with gnomAD scores).
Sufficient evidence (≥4 layers): 19,946 → 20,683 genes.
Validation median percentile: 79.9% → 81.1%.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

data/report/candidates.provenance.yaml

@@ -1,12 +1,12 @@
-generated_at: '2026-02-15T21:13:11.954116+00:00'
+generated_at: '2026-02-15T21:49:14.608460+00:00'
 output_files:
 - candidates.tsv
 - candidates.parquet
 statistics:
-  total_candidates: 21103
-  high_count: 18
-  medium_count: 9577
-  low_count: 11508
+  total_candidates: 21177
+  high_count: 82
+  medium_count: 9626
+  low_count: 11469
 column_count: 22
 column_names:
 - gene_id