Raise record contract to 2.0

[nikhilvdev]

This change bumps the minimum Python version to 3.10 and updates the log record contract to schema_version 2.0. It adds the shared JSON Schema and golden-record fixtures, a backward-compatible parse_record() loader for 1.x/2.x records, and the new top-level llm block plus reserved meta fields. The changelog, migration guide, formatter/parser updates, and New Relic transport handling are updated to keep the Python package aligned with the JS contract and catch schema drift in CI.

Summary

Checklist

• ruff check . passes
• mypy logquill passes (--strict on the public API)
• pytest passes, and new surface area has test coverage
• Public API additions are documented in README.md with a runnable example
• CHANGELOG.md has an entry under Unreleased
• If this touches the cross-language contract (levels, logger methods,
  record shape, transport/plugin contracts, config shape) shared with
  logquill-js, a matching tracking issue/PR is opened there

Scope