Canonicalize integers as doubles in RFC 8785 mode

[ydah]

RFC 8785 §3.2.2.3 requires ECMAScript serialization of double-precision numbers. Integer generation currently bypasses the canonical number formatter.

JSON.generate(9007199254740993, rfc8785: true)
# Before: "9007199254740993"
# After:  "9007199254740992"

JSON.generate(10**21, rfc8785: true)
# Before: "1000000000000000000000"
# After:  "1e+21"

Route integers through the existing formatter after converting to Float. Reject integers outside the finite Float range with GeneratorError before conversion, avoiding overflow warnings. Ordinary integer generation remains unchanged.