Dreamcast Port Initial Implementation

[h4mu]

This PR adds initial support for porting Rise of the Triad to the Sega Dreamcast.

Key changes:

1. Platform Abstraction: Updated rott/rt_def.h to include PLATFORM_DREAMCAST support, including POSIX-compatible header inclusions and path definitions.
2. Read-Only Filesystem: Modified core I/O functions in rott/rt_util.c, configuration saving in rott/rt_cfg.c, and WAD writing in rott/w_wad.c to disable write operations when building for Dreamcast, as the platform typically runs from read-only media (GD-ROM).
3. Submodules: Integrated KallistiOS (KOS), a Dreamcast-specific SDL2 fork, and the official SDL2_mixer as git submodules in the dreamcast/ directory.
4. Development Environment: Provided a Dockerfile and a setup_env.sh script to automate the setup of the sh-elf cross-compiler and the compilation of KOS and its port libraries.
5. Build System: Added a dedicated Makefile in dreamcast/ that correctly handles the unique build process for Dreamcast ELFs across all game editions (Shareware, Registered, CD, and Site License).

These changes are isolated behind the PLATFORM_DREAMCAST macro to preserve compatibility with existing platforms.

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PR created automatically by Jules for task 6818930104015367374 started by @h4mu

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