Use consistent form data limit naming.

[samuel-williams-shopify]

Summary

• rename form-data parser constraints to use the measured quantity followed by _limit
• use size_limit for the single encoded representation and preserve pair_count_limit and depth_limit as distinct constraints
• align constants, keywords, internal state, tests, documentation, and release notes

Testing

• bundle exec bake test
• bundle exec rubocop
• COVERAGE=PartialSummary bundle exec bake decode:index:coverage lib