Preserve float precision in RFC 8785 number formatting

[ydah]

RFC 8785 §3.2.2.3 requires ECMAScript number serialization. The current "%.15E" formatting limits numbers to 16 significant digits and can change the underlying double value.

0.1 + 0.2 is not an RFC test vector, but ECMAScript serializes its result as 0.30000000000000004. Float::MAX is covered by Appendix B.

Before

JSON.generate(0.1 + 0.2, rfc8785: true)
# => "0.3"

JSON.generate(Float::MAX, rfc8785: true)
# => "1.797693134862316e+308"

After

JSON.generate(0.1 + 0.2, rfc8785: true)
# => "0.30000000000000004"

JSON.generate(Float::MAX, rfc8785: true)
# => "1.7976931348623157e+308"

This replaces "%.15E" with Float#to_s while retaining ECMAScript notation formatting. On CRuby, all Appendix B vectors pass, and 10,000 random finite doubles match Node.js and round-trip correctly.

The original implementation also has an open PR addressing the same issue: dryruby/json-canonicalization#6.