Normalize JSON in one module

[myabc]

🤖 Normalization was spread over Helpers, Exclusion and BeJsonEql, and the key sort that makes equivalence work was hidden inside Exclusion#exclude_keys. As a result "normalized JSON" meant two different things: normalize_json kept source key order while be_json_eql compared sorted keys, and include_json compared Ruby values rather than normalized JSON.

What changes

• New internal JsonSpec::Normalization.normalize(ruby, excluded_keys: []) owns dropping excluded keys, sorting keys and generating text.
• normalize_json, generate_normalized_json, be_json_eql and include_json all go through it, so they agree on what "equivalent" means.
• JsonSpec::Exclusion shrinks to the per-matcher set of excluded keys.
• Adds CONTEXT.md, a short domain glossary defining "normalized JSON" and "excluded keys".

Behaviour changes

All are listed in CHANGELOG.md under 2.0.0 unless noted.

• normalize_json and generate_normalized_json now sort object keys. They still exclude nothing.
• include_json compares normalized JSON element by element, like be_json_eql. An integer no longer matches an equal float ([10.0] does not include 10), and a non-string expected value against a JSON string fails instead of raising TypeError.
• JsonSpec::Exclusion.exclude_keys and exclude_key? are removed, and the module no longer extends itself. It was never documented.
• Not in the changelog: normalization generates with allow_nan: true, because JSON.parse reads numbers too large for a float (1e400) as Infinity. Without it include_json would raise on any document containing such a number. This also means be_json_eql and normalize_json no longer raise JSON::GeneratorError on such input, and normalize_json("1e400") returns Infinity, which is not valid JSON. Open question for review: keep this, or narrow it to include_json only.

Testing

• bundle exec rake passes locally on Ruby 4.0.2: 129 examples, 53 scenarios.
• New spec/json_spec/normalization_spec.rb tests the module at its interface. New specs cover key sorting in the helpers, the include_json equality changes, overflowing numbers, and requiring only json_spec/helpers.
• A Codex adversarial review found two regressions (direct require "json_spec/helpers" raising NameError, and the overflowing-number crash); both are fixed with regression specs. The review was not re-run after the fixes.