Fix reversed snpid allele order in VEP tutorial

[buyan-kh]

Summary

• Fixes Ref and alt reversed in VEP tutorial #33: in the VEP / variant-effect tutorial, snpid values looked inconsistent with the ref/alt columns.
• Confirmed against dbSNP/NCBI that the CSV ref/alt columns match the hg19 reference; the mismatch came from GWAS-style chrom:pos_ALT_REF snpids (and alt listed before ref).
• Tutorial now rebuilds each snpid as chrom:pos_REF_ALT and reorders columns to the usual ref, alt order after load. Stored notebook outputs updated accordingly.
• No library code changes — grelu.variant already treats ref/alt correctly.

Test plan

• Cross-checked several tutorial variants (e.g. rs138940043, rs66957194) against NCBI: genome REF matches the ref column, not the first allele in the old snpid.
• Confirmed no open PR already targeting Ref and alt reversed in VEP tutorial #33.
• Spot-check rendered docs/tutorials/5_variant.ipynb tables: snpids read …_REF_ALT and align with ref/alt.