optimize single-pass f32 parsing

[liuq19]

Summary

• add a dedicated single-pass f32 parser in sonic-number
• use that parser from Deserializer::deserialize_f32
• lock in overflow handling with explicit negative tests returning FloatMustBeFinite
• strengthen fuzz coverage with a dedicated f32 literal target, curated seeds, and CI fuzz-time normalization

Background

The previous f32 deserialization path first parsed the number as f64 and then converted it back to f32. For tie-boundary inputs such as 17005001.000000000000130, that can produce an off-by-one ULP result.

Changes

This updates f32 parsing to stay on a single pass, avoids the extra string reparse fallback, and preserves the expected f32 rounding behavior for affected cases.

On top of the parser change, this PR also strengthens regression coverage around f32 parsing:

• add direct regression tests for precision-sensitive literals and overflow-to-infinity cases such as 3.4028236e38 and 1e39
• add a dedicated fuzz_f32_literal target with curated corpus seeds for rounding and overflow boundaries
• expand fuzz seed coverage for JSON value, string, path, deep nesting, and serde roundtrip paths
• normalize FUZZ_TIME in CI so values like 3m are passed to libFuzzer as seconds instead of effectively running for only a few seconds

Validation

• cargo test test_deserialize_f32_single_pass_precision
• cargo test --manifest-path sonic-number/Cargo.toml test_parse_float32
• cargo test --manifest-path fuzz/Cargo.toml

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.16%. Comparing base (96daa50) to head (890e228).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #227      +/-   ##
==========================================
+ Coverage   71.08%   71.16%   +0.07%     
==========================================
  Files          42       42              
  Lines        9744     9762      +18     
==========================================
+ Hits         6927     6947      +20     
+ Misses       2817     2815       -2     

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[Copilot]

Pull request overview

This PR updates f32 deserialization to avoid the previous f64-first parsing path (which could mis-round tie-boundary decimals), by introducing and wiring in a dedicated single-pass f32 parser in sonic-number.

Changes:

• Add sonic_number::parse_float32 and shared generic float-parsing helpers for f32.
• Switch Deserializer::deserialize_f32 to use the new single-pass f32 parser (removing the raw-string fallback parse).
• Add regression tests for the tie-boundary precision case and direct parse_float32 unit tests.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/serde/mod.rs	Adds a regression test covering the precision edge case and -0 handling for f32 deserialization.
src/serde/de.rs	Replaces the prior f64→f32 + fallback parse approach with a dedicated single-pass f32 parse call.
src/parser.rs	Adds a Parser::parse_float32 wrapper that advances the reader index appropriately and maps number errors into parser errors.
sonic-number/src/lib.rs	Implements parse_float32, introduces generic float helpers, and adds unit tests for parse_float32.

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