Reduce converter string overhead in hot paths

[Copilot]

This PR applies targeted micro-optimizations in the converter to reduce avoidable allocations/copies and use cheaper character APIs where applicable. Scope is intentionally narrow: no behavioral changes, only lower-overhead string handling.

• Whitespace parsing: use char overloads

  • Replaced single-character string literals with character literals in trim logic to use the char overloads directly.
  • Affected code path: format width normalization in GetFmtArg.

• Trait generation: avoid copying large string fragments

  • Updated ConvertOrdAndPartialOrdTraitsBase to accept std::string_view instead of owning std::string parameters.
  • This removes repeated by-value copies of prebuilt branch/return fragments during trait emission.

• Code emission utility: reduce variadic argument copies

  • Updated Converter::_StrCat variadic parameters from pass-by-value to const&.
  • This avoids unnecessary temporary/string copies during high-frequency emission calls.

// Before
fmt_width.erase(0, fmt_width.find_first_not_of(" "));
fmt_width.erase(fmt_width.find_last_not_of(" ") + 1);

// After
fmt_width.erase(0, fmt_width.find_first_not_of(' '));
fmt_width.erase(fmt_width.find_last_not_of(' ') + 1);