fix: restore gnomAD and expression evidence layers for complete 6-layer scoring

Three bugs prevented gnomAD and expression data from contributing to scores:
1. gnomAD COLUMN_VARIANTS mapped "gene" (HGNC symbol) to gene_id instead of
   gene_symbol, causing JOIN miss with gene_universe (Ensembl IDs)
2. Expression HPA data was fetched but never merged (lf_hpa unused)
3. GTEx versioned Ensembl IDs (ENSG*.5) didn't match gene_universe

Results: gnomAD 78.5% coverage, expression 87.4%, 19946 genes with ≥4 layers.
HIGH tier refined from 44 → 18 candidates. Validation PASSED (CDH23 96.5th pctl).

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

data/report/candidates.provenance.yaml

@@ -1,12 +1,12 @@
-generated_at: '2026-02-15T20:47:53.707513+00:00'
+generated_at: '2026-02-15T21:13:11.954116+00:00'
 output_files:
 - candidates.tsv
 - candidates.parquet
 statistics:
-  total_candidates: 19342
-  high_count: 44
-  medium_count: 7268
-  low_count: 12030
+  total_candidates: 21103
+  high_count: 18
+  medium_count: 9577
+  low_count: 11508
 column_count: 22
 column_names:
 - gene_id