Make parse_csv_float locale-independent (replace strtof with istringstream + C locale)

[Copilot]

parse_csv_float used std::strtof, which is LC_NUMERIC-dependent and can misparse decimal separators in non-C locales.

Changes

• Locale-safe parsing: replaced std::strtof with std::istringstream imbued with std::locale::classic(), ensuring . is always the decimal separator regardless of the process locale
• Reuse stream/buffer across iterations: istringstream and the token std::string are hoisted outside the loop; reused via ss.clear() + ss.str() + token.assign() to avoid per-iteration reconstruction
• Strict token validation: added !ss.eof() check after extraction — tokens like 3.14abc now break the loop rather than silently accepting the numeric prefix
• Includes: removed <cerrno> and <cstdlib> (no longer needed); added <locale> and <sstream>

std::istringstream ss;
ss.imbue(std::locale::classic());
std::string token;
// ...per iteration:
token.assign(first, token_end);
ss.clear();
ss.str(token);
float v{};
if (!(ss >> v) || !ss.eof()) { break; }