Iris C++ 0.1.0

• Add the C++23 library iris, a native port of Gecode 6.5.0's reader that
  does not depend on Gecode. iris::Spec parses and loads reader specs and
  reads and catalogs data files and text; each operation that throws
  iris::Error has a try_ variant that returns std::expected.
• Add iris::Data, whose accessors return std::span rows and
  std::string_view strings, iris::ValueRef for inspecting values without
  misuse, and Data::dzn() for MiniZinc data. Reading enforces the work
  budgets of the Iris Language Specification.
• Give iris::Error every part of a diagnostic, from the code and locations
  to the excerpt, context, record path and earlier rejection, as
  iris::ErrorDetails, and render the diagnostic of the Specification from
  what().
• Allocate from a std::pmr::memory_resource that the caller supplies, so
  that a host such as Gecode can give the library its heap.
• Add typed handles, iris::Field, which data.get and params.set take,
  and print them for any reader spec with iris gen --lang cpp.
• Add the spec builder, <iris/build.hpp>, which writes reader specs from C++
  statements, checks pattern placeholders at compile time, and names the
  std::source_location of the statement in its errors and in the errors of
  reading a built spec.
• Install a CMake package that find_package(iris) finds, with the target
  iris::iris, and support FetchContent and add_subdirectory. Each release
  publishes a source archive, iris-cpp-VERSION.tar.gz, with its SHA-256
  checksum.
• Test with GCC 14, Clang 20, Apple Clang and MSVC, under ASan, UBSan and
  TSan, with libFuzzer, and against the whole conformance suite.