Migrate ChkX operations to Boost int128

[Copilot]

ChkNat and ChkInt now use Boost’s fixed-width int128_t instead of compiler-specific __int128_t, while retaining existing error and saturation behavior.

• Runtime representation

  • Replace native 128-bit types across arithmetic, parsing, emission, literals, and string conversion.
  • Add no-exception Boost hooks compatible with the runtime configuration.

• Behavior preservation

  • Retain npos propagation and overflow saturation.
  • Guard multiplication before evaluating potentially overflowing expressions.
  • Preserve ChkNat underflow and division-by-zero errors.

• Compiler layouts

  • Update ChkX metadata for Boost’s 32-byte representation.
  • Support copying ChkX-containing generated unions.

• Coverage

  • Add cases for positive and negative 124-bit saturation.
  • Add coverage for ChkInt values embedded in concept unions.

• Fixes Shift ChkX support to boost #504

[marron-at-work]

Not willing to pay 4 bytes per ChkX number. Closing this path.