[codex] Add Isovar mutant transcript source

[iskandr]

Summary

• Replace the old combined VarcodeAdapter with IsovarMutantTranscript, a focused implementation of varcode MutantTranscriptSource protocol.
• Keep read phasing on IsovarReadPhasing and make the split providers compose safely through ordinary multiple inheritance.
• Update tests for the new protocol shape, including b16 fixture coverage, the plain multiple-inheritance regression case, and the end-to-end tumor.effects(phase_resolver=...) acceptance path.
• Bump isovar to 1.7.0 and require varcode>=5.0.0.

Dependency

Requires varcode 5.0.0, which is now released on PyPI: https://pypi.org/project/varcode/5.0.0/

Refs #182 and #184.

Validation

Ran locally against PyPI-installed varcode 5.0.0 from site-packages:

• uv pip install varcode==5.0.0
• pytest tests/test_mutant_transcript_source.py tests/test_transcript_edits.py::test_isovar_mutant_transcript_includes_unexplained_edits tests/test_read_phasing.py::test_docstring_examples_run - 8 passed
• ./lint.sh
• ./test.sh - 214 passed

[coveralls]

coverage: 93.768% (+0.2%) from 93.592% — codex/mutant-transcript-source into master