Fix BatchWrite affinity and IPv6 routing URLs

[dkropachev]

Summary

• make BatchWriteItem ANY_WRITE votes use the first pick from the canonical seeded LazyQueryPlan instead of modulo selection
• bracket raw IPv6 seed/live-node addresses when constructing discovery, initial endpoint, and retry-plan URLs
• add regression coverage for canonical BatchWrite voting and IPv6 URL formatting

Verification

• uv run pytest tests/unit -q
• uv run ruff check alternator tests/unit examples
• uv run ruff format --check alternator tests/unit examples
• uv run mypy alternator examples tests/unit --strict
• uv run python -m compileall -q alternator tests examples