Add unified rfl::Settings::with<>() API for format-specific settings

[Centimo]

Summary

Replaces ad-hoc with_delimiter() / with_compression() / etc. accessors on csv::Settings and parquet::Settings with a single uniform with<>() accessor injected by the RFL_SETTINGS_OPS(Derived) macro. Two overloads are provided:

• with<&T::field>(value) — pointer-to-member.
• with<"field">(value) — field name as string literal.

Both return a copy with the chosen const field replaced. Old per-field accessors are removed; existing tests are updated.

Motivation

The previous design required hand-written with_*() per field, growing linearly with each new option. The new macro replaces that boilerplate with a single line, makes adding fields a one-liner, and is consistent across formats.

Const fields

All data members in csv::Settings and parquet::Settings are now declared const. Settings are intended to be configured once and consumed; preventing accidental in-place mutation makes that intent explicit at the type level. The with<>() accessor returns a fresh copy with one field replaced, so the configurable surface is unchanged. RFL_SETTINGS_OPS enforces this — it rejects pointers-to-non-const members, so any future field added without const is a compile error rather than silent regression.

Implementation notes

• field_index_v<T, FieldPtr> recovers the field index in a consteval context by comparing &(fake_object<T>().*FieldPtr) against the addresses of the structured-binding fields. The index then feeds back into the existing fake-object name-lookup path (form &fake.field), which MSVC parses correctly — direct PTM literals do not.
• field_index_by_name_v<T, Name> does the analogous lookup for the string-literal overload.
• get_fake_object.hpp switches from a static const member of a struct template to an extern const variable template (matching Boost.PFR). Without this, GCC requires the fake-object symbol at link time when evaluating &(fake.*FieldPtr) in consteval.

Tests

tests/cli/test_settings_macro.cpp covers single-field replace, chained replace by PTM, chained replace by name, mixed PTM+name chains, and string-move semantics. Existing csv/parquet tests are updated to use the new API. JSON 214/214 and CLI 46/47 pass locally (1 skipped is an unrelated locale test).

[gemini-code-assist]

Code Review

This pull request introduces a unified with<>() accessor for settings structs via the RFL_SETTINGS_OPS macro, supporting both pointer-to-member and string literal template arguments. This replaces individual with_field methods in CSV and Parquet settings and promotes immutability by transitioning fields to be const. The review feedback identifies that the string-literal overload of with<>() does not currently enforce the const requirement on fields, unlike the pointer-to-member version, and suggests adding a static_assert to ensure consistent enforcement of immutability.

[gemini-code-assist]

The with<"name">() overload doesn't enforce that fields in a Settings struct must be const, unlike the pointer-to-member overload with<&T::field>(). This could lead to mutable settings fields being used, which goes against the design goal of this PR.

To ensure consistency and enforce immutability for all Settings fields, I suggest adding a static_assert to check that the field is const.

  constexpr std::size_t I = field_index_by_name_v<T, Name>;
  static_assert(I != static_cast<std::size_t>(-1),
                "No field with the given name exists in T.");
  using FieldTypeWithCv = std::remove_pointer_t<
      decltype(get_ith_field_from_fake_object<T, static_cast<int>(I)>())>;
  static_assert(std::is_const_v<FieldTypeWithCv>,
                "Fields in Settings structs must be const.");
  return rfl::replace(_obj, rfl::make_field<Name>(std::move(_value)));