perf(json): parse directly from string buffers

[SaveTheRbtz]

Summary

• Parse JsonParser input with JsonCpp CharReader directly from the existing string buffer.
• Keep \x escape compatibility, but only allocate and run regex_replace when that escape is present.

Why this was needed

baseline.perf.data showed the JSON utility paying a large common-case regex tax before parsing:

• std::regex search/replace: about 1.10B child cycles
• std::regex::_M_dfs: 743M self cycles
• Json parse path: 834M child cycles

Change

Parse from the existing string buffer with JsonCpp's CharReader. Preserve the \x compatibility repair, but first check whether the escape is present so the common path avoids allocating a modified string and running regex replacement.

Effect in comparison

The regex path disappears from visible hot symbols. Json parse work drops from 834M to 188M cycles, and the overall JsonCpp bucket drops from roughly 700M to 186M cycles.

Testing

• ninja -C build
• meson test -C build

[SaveTheRbtz]

cc @zjeffer @xander1421 @Alexays, you look like the most relevant reviewers from git history:

• @zjeffer added the \x -> \u00 compatibility path and JsonParser tests in Fix JSON parsing with hexadecimal characters #2496.
• @xander1421 recently changed JsonParser to use a local CharReaderBuilder for thread safety in fix(json): use local CharReaderBuilder for thread safety #4789.
• @Alexays reviewed/merged both related parser changes and has the original parser/module history.