Effect strings in CSV: SpliceOutcomeSet hides alternatives, REF==ALT records become Silent p.335=, 1-bp deletions use a range

Reproduce the examples with EffectCollection.to_dataframe(), effects.top_priority_effect_per_variant(), varcode --one-per-variant, load_vcf, and varcode-genes using the commands and VCF records in the issue. Trace how SpliceOutcomeSet, no-op variants, single-nucleotide deletions, and structural variants are represented before export; done means each listed CSV or HGVS behavior matches the stated expected result.

Effect strings in EffectCollection.to_dataframe() / CSV (8.0.0): SpliceOutcomeSet hides its alternatives, REF==ALT records become "Silent", and single-base deletions use a range.

From varcode --vcf tests/data/somatic_hg19_14muts.vcf --output-csv out.csv on main at 50eafde:

1. SpliceOutcomeSet exports only its first candidate, and top-priority selection drops the rest.

   chr1 g.53513530A>C,...,ENST00000371514,SCP2-001,SpliceOutcomeSet,splice-set:normal-splicing (p.D490A)
   

   The effect has four candidates: normal-splicing (p.D490A), exon-skip ENSE00003489518 (p.D490fs), cryptic-donor (predicted) and intron-retention (donor side, predicted). The CSV shows only the first. effect_type is the container class name, not a consequence. effects.top_priority_effect_per_variant() (and varcode --one-per-variant) turns the same transcript into Substitution p.D490A, so the possible frameshift vanishes. Nine rows in this 14-variant file are SpliceOutcomeSet. Related to #421 and #423.

2. A REF==ALT record is annotated as Silent with a malformed protein change. The VCF line chr11 118244286 . G G produces:

   chr11 g.118244286,11,118244286,,,False,False,False,ENSG00000110344,UBE4A,ENST00000252108,UBE4A-001,Silent,p.335=
   

   The HGVS strings g.118244286 and p.335= have no alleles and no residue. varcode-genes lists the same record with ref=G, alt=G. Expected: load_vcf drops or warns about no-op records, or the effect is reported as NoChange, not Silent.

3. Single-nucleotide deletions use a one-base range. --variant 17 36731197 C . gives chr17 g.36731197_36731197delC. HGVS for a one-nucleotide deletion is g.36731197del (or delC), with no _ range.

4. Structural variants carry placeholder alleles in the dataframe. See #417. to_dataframe() no longer crashes, but SV rows still show alt=A and is_snv=True.