Build dat samples in their own clang pipeline

[ribbanya]

The dat samples no longer go through dtk, the DOL or MWCC: the archives weren't built with MWCC, so matching them only needs a pipeline that compiles to the same bytes. They now build in their own CMake preset, dat, in build/GALE01/dat, which is also their objdiff project.

• cmake/Dat.cmake: per archive, four ninja steps: slice the archive into a target object (samples slice), generate C from it (samples codegen), format it with the repository's .clang-format, and compile it with the DWARF build's flags. samples project writes objdiff.json last. compile_commands.json gives clangd the same flags.
• The DWARF build is per game version (MELEE_VERSION).
• types export writes a compact, deduplicated types file (postcard, 1.8 MB), so each step loads the types in milliseconds instead of reading the 160 MB ELF.
• Samples are per archive and named as the archive names them (its public symbol, else x<OFFSET>). The generated C is a directory per archive with a header and source per root; a unit file includes the sources. Pointers between roots go through the headers' extern declarations.
• Union objects are declared as the member their tag chose; generated comments are Doxygen.
• configure.py, the root objdiff project and melee-dtk know nothing of dats; config/GALE01/dat.yml loses its samples section.
• nix: melee-dat-samples is the CMake DWARF build with the samples on.
• HSD_RObjDesc_u is back in its original member order: designated initializers don't need a pointer member first.

11237 of 11239 samples match; the two that don't are in TODO.md.