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fix(json): parse big integer literals to correctly rounded doubles

lex_integer_end conflated "cannot be represented exactly in 2^53" with
"out of double range": once the exact-integer accumulator overflowed
SAFE_INTEGER_LIMIT it returned the Number(Infinity, repr=...) sentinel
immediately, without ever consulting strconv. The decimal/exponent path
already falls back to strconv for correct rounding, so the same value
parsed differently depending on spelling ("10000000000000000000" gave
Infinity while "1e19" gave 1.0e19).

On overflow, fall back to @internal/strconv.parse_double for the
correctly rounded value and keep the exact source text in repr, so
stringify remains lossless. The infinity sentinel now only applies to
literals strconv itself rejects (beyond double range), matching how
mainstream implementations behave (JS rounds; serde_json/Jackson/
Python keep exact or rounded values; nobody yields Infinity in range).

This makes the red tests from the previous commit pass, fixes Eq
collapsing all big-integer texts into one equivalence class, and lets
from_json : Double accept in-range values, while the existing
out-of-range sentinel tests keep passing unchanged.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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