[codex] Add varcode assembly provider adapter

[iskandr]

Summary

• add isovar.varcode_adapter.VarcodeAdapter, a dedicated adapter that presents existing IsovarResult collections through varcode's IsovarAssemblyProvider protocol
• project transcript-keyed Translation objects into varcode MutantTranscript values so IsovarPhaseResolver can attach contig-derived proteins to effects
• bump the varcode dependency floor to >=4.15.0 and add integration tests against the real varcode.phasing runtime protocol

Notes

• variants_in_contig() currently exposes already-known phased RNA variants for the transcript-specific contig; it does not yet enumerate germline SNPs or unexplained contig edits directly from assembled cDNA
• downstream coordination is tracked in Adopt varcode 4.x for isovar 1.5.0 assembly-provider integration vaxrank#238 because current vaxrank still pins varcode<3

Validation

• ./lint.sh
• ./test.sh

Closes #160

[coveralls]

coverage: 93.305% (-0.09%) from 93.391% — add-varcode-assembly-provider into master