Modernize the macOS build: universal, macOS 15+, Steam

[TomBrouws]

Build DROD on modern macOS with Apple Clang + Homebrew, producing a self-contained, ad-hoc-signed .app/.dmg that runs on macOS 15+ (Intel and Apple Silicon). Replaces the unmaintained gcc-4.9 / x86_64 Intel setup.

Build system (Master/Darwin/Config, Makefile):

• Apple Clang, C++11, arm64 by default; UNIVERSAL_BUILD=1 builds x86_64+arm64.
• Module source lists come from the maintained .2019.vcxproj files instead of the stale VC6 .dsp files.
• -iquote for module includes so macOS's case-insensitive filesystem doesn't resolve system <wchar.h> to BackEndLib/Wchar.h.
• Build types: custom, release, debug, and steam (-DSTEAMBUILD -DSTEAMBUILD_TSS_APP, links libsteam_api). Archive rule recreates fat .a files so incremental universal builds don't trip macOS ar.

Dependencies (Master/Darwin/build_deps.sh):

• All third-party libs are built from source as universal dylibs targeting macOS 15 into Master/Darwin/deps (curl + zlib come from the SDK): real SDL2 (no sdl2-compat/SDL3), lean SDL2_mixer (Ogg/Vorbis + WAV + drmp3), SDL2_ttf, freetype, libpng, jpeg-turbo, expat, ogg/vorbis/theora, jsoncpp, and metakit. This is what lets the app back-deploy to macOS 15 / Intel.

Source fixes for libc++/clang (guarded, portable):

• BackEndLib/Wchar.cpp, DRODLib/I18N.cpp: avoid char16_t string streams, which libc++ lacks the std::ctype<char16_t> facet for.

Packaging (Master/Darwin/build_macos.sh):

• One command builds the bundle: installs build tools, builds deps + drod, assembles the .app, vendors all dylibs via dylibbundler, and ad-hoc signs.
• Bundle structure comes from the repo (stub drod5_0.dat, Help, Licenses, icon, Info.plist); loose Fonts/Bitmaps are overlaid from the drodData repo.
• Real game data is copied from --dat-dir or a gitignored Master/Darwin/localdata (all *.dat); nothing copyrighted is committed.
• Steam: --steam builds the Steam app and vendors libsteam_api from the SDK; a gitignored steam_appid.txt (via --steam-appid) can be dropped in for out-of-Steam dev testing.

Also: remove unused SDLMain.{h,m} and buildcmd.sh; replace the stale OSX docs with CompilingDrod_macOS.md; update readme and the Info.plist copyright; ignore build output and local dev files.

[TomBrouws]

@mrimer we'll need CaravelGames/metakit#1 to be merged as well and the submodule updated to make this work.