Convert the floats of json_view from the digit layout

[nlohmann]

Part of the stack for the zero-copy view (#5295).

Summary

While parsing, the view records where the integer digits, the fraction digits, and the exponent of a float token are. For floats and doubles with at most 19 digits, the value is now read from that layout:

• the digits are read eight at a time, without scanning the token;
• the result is rounded by the library's conversion core, decimal_to_float() from Speed up the lexer: own float parser, string scan, and \u table #5738: Clinger's fast path where both operands are exact, and Eisel-Lemire otherwise, which needs no fallback for up to 19 digits.

Both round correctly, so the values are those of parse(). Longer tokens, and long double, keep the library's conversion of the whole token.

get<double>(), materialize(), dump(), and the comparisons all use it.

Benchmarks

Traversing canada.json (111,000 floats, every number converted): 0.95 → 1.29 GB/s. The baseline already includes #5738, whose converter the view uses before this PR.

Tests

Tokens around the limits (19 and 20 digits, 2^53, 10^22, underflow, and those of float) join the bit-for-bit comparison with parse().

Public API

No change.

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Written by Claude Code.

🤖 Generated with Claude Code

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[nlohmann]

The zero-copy view stack (#5295) was rebased onto develop and regrouped from 20 PRs into 10, one squashed commit each. The changes of this PR are now in #5624; its review comments stay here for reference.

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Written by Claude Code.