Use Parsers 3 kernels for number parsing

[quinnj]

Summary

This draft updates JSON number parsing for Parsers 3 while retaining compatibility with Parsers 1 and 2 and Julia 1.9.

• Keep strict JSON number grammar in JSON.jl.
• Use span-exact Parsers 3 kernels for built-in numeric targets.
• Keep adaptive untyped promotion: Int64 to BigInt, and Float64 overflow to BigFloat.
• Parse typed Float32 and BigFloat values from the original token bytes. This removes intermediate Float64 rounding.
• Add explicit CI and coverage lanes for Parsers 1.1.2 and the exact head of Rewrite Parsers around span-exact scalar kernels JuliaData/Parsers.jl#210.

This branch is stacked on #479 because it extends that number-parsing change. The first three commits come from #479. They can drop from this PR after #479 merges.

Compatibility design

The implementation selects a bounded adapter at compile time:

• Parsers 3 uses parseint, parsebigint, parsefloat, and parsenext on exact byte spans.
• Parsers 2 keeps its exact-span integer adapter.
• Parsers 1 keeps its older xparse integer adapter.
• Parsers 1 and 2 use Base for validated finite floats and wide integers. This avoids known legacy parser crashes, incorrect long-decimal results, and pooled BigInt ownership.

The package still declares Parsers = "1, 2, 3" and julia = "1.9". Parsers 3 itself requires Julia 1.10 or later, so Julia 1.9 resolves a compatible older Parsers release.

Observable corrections

• typemin(Int64) now stays an Int64.
• allownan=true adds configured and native special spellings. It no longer coerces every finite number to Float64.
• Finite numbers with a leading + are rejected as non-standard JSON.
• Negative zero keeps its sign.
• Long finite decimals use Base-compatible rounding on the Parsers 1 and 2 compatibility paths.
• Typed Float32 and BigFloat values no longer double-round through Float64 on Parsers 3.
• Returned BigInt values have independent storage.

Validation

Independent tests used the exact branch commit and exact Parsers 3 pin:

• Focused grammar, span, rounding, promotion, special-value, and ownership matrix: 657,429/657,429 across seven lanes, with zero Float64 oracle differences.
• Parsers 1 / Julia 1.9: 61,472 passed.
• Parsers 2 / Julia 1.9: 61,472 passed.
• Parsers 2 / Julia 1.10: 61,472 passed.
• Parsers 2 / Julia 1.12: 61,477 passed, including trim 6/6.
• Parsers 3 / Julia 1.10: 61,479 passed.
• Parsers 3 / Julia 1.12: 61,484 passed, including trim 6/6.
• Julia 1.14 nightly: all 61,478 non-trim checks passed. JuliaC 0.3.8 alone could not trim that nightly because of unresolved new Base CancelTokenKey calls.
• Documentation builds successfully.

Performance

Against the stacked #479 Parsers 2 baseline on Julia 1.12.6:

• Common Int64: within noise to 6% faster.
• Common Float64: 21% to 24% faster.
• Wide BigInt: 14x to 18x faster, with about 32x fewer allocations.
• Typed Float32: about 3.2x faster and correctly rounded.
• Float64-overflow to BigFloat: within noise to 7% faster, with one fewer allocation.

Parsers 1 and 2 byte-vector float inputs require one bounded temporary String. This is an intentional compatibility cost for correct rounding and safe behavior. Parsers 3 parses the original span directly.

Dependency state

The Parsers 3 CI lane pins exact green draft head 83c7142fb714cb87261ef38eec7ab103444eb30d from JuliaData/Parsers.jl#210. This pin should move to the released Parsers 3 version before this PR merges.

Co-authored by Codex

[codecov]

Codecov Report

❌ Patch coverage is 93.36735% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.78%. Comparing base (233d644) to head (3445472).

Files with missing lines	Patch %	Lines
src/parse.jl	84.21%	9 Missing ⚠️
src/lazy.jl	97.12%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #480      +/-   ##
==========================================
+ Coverage   89.99%   90.78%   +0.79%     
==========================================
  Files           7        7              
  Lines        1519     1628     +109     
==========================================
+ Hits         1367     1478     +111     
+ Misses        152      150       -2     

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