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Closes #5184

This PR updates the default Newick branch length format from %1.5f to %1.8g.

The previous default could silently round small nonzero branch lengths to 0.00000 when writing Newick trees, which can lose biologically meaningful distance information. Using significant digits preserves small values such as 0.000001 while avoiding unnecessarily long fixed-width decimal output for all branch lengths.

Changes included:

• Update the default format_branch_length used by the Newick writer.
• Add regression coverage for small nonzero branch lengths.
• Update the expected Newick output for zero-length branches under the new default format.

Testing:

cd Tests
python -m pytest test_Phylo.py
python -m pre_commit run --files Bio/Phylo/NewickIO.py Tests/test_Phylo.py

Result:

39 passed, 1 warning
pre-commit passed