Draft: native D3D12 renderer and reproducible contributor handoff

[NeverCookFirst]

Adds an opt-in C++ native D3D12 renderer that intercepts the recompiled game’s graphics calls and uses ahead-of-time translated shader archives. LEGODIMENSIONS_NATIVE_GPU remains OFF by default; this is an experimental collaboration draft.

Included work

• Native device/presentation, shader and placement lookup, vertex/index/texture uploads, render state, depth/resolve handling, queries and synchronization, plus bounded diagnostic tools.
• Texture-channel and DXT3A handling, alpha/viewport and gradient compiler preparations, native DoF handling, Alt+Enter borderless toggling, and the latest polygon-offset pipeline change.
• A tracked contributor handoff at tools/gpu-aot/COLLABORATION.md, pinned compiler reconstruction, and a standalone CMake/CTest target with 19 asset-free helper checks and Ubuntu CI.
• The exact SDK gitlink is published in dependency draft Expose SDK dependency for experimental native renderer collaboration rexglue-sdk#1. Its committed head is 17e3675767067339581018c7e722e1d8d65de201; uncommitted SDK/dependency work is excluded.

Verified results

• Windows native game target built with Clang 20.1.8, MSVC 14.44 headers/libs and Windows SDK 10.0.26100.0.
• All 19 standalone helper checks passed locally on Windows and on Ubuntu CI with GCC 13.3.0, with assertions enabled in Release. Successful Ubuntu run.
• Applying the tracked shader adaptation to pinned XenosRecomp 339af41df2c23dbe3256c1c377716b81a0e0fe6b reproduced the local patched checkout. The alpha → viewport → gradient preparation chain reproduced all ten current compiler C++/header inputs after newline normalization; the shared ABI remains 624 bytes.
• The October4 local installed archive audit recorded main 9290 shaders/18444 linked variants, runtime 255/458, and 18993 placement-index records. These generated archives are excluded from this PR.

Linux and input limitations

• The native game renderer cannot currently be built on Linux: game CMake explicitly rejects native GPU mode outside Windows, and the implementation uses Win32/D3D12.
• The current AOT compiler preparation links Windows DXC .lib/.dll files and Windows dependency outputs. Native probe scripts use PowerShell/Windows APIs. No Linux native game, Wine, cross-compilation, or shader-compiler build was validated for this handoff.
• Linux contributors can run the asset-free helper target and review/edit source. Its CI result is reported in GitHub checks; it does not validate the renderer’s image or game performance.
• A clean clone lacks generated game C++, generated/rexglue.cmake, game data and shader banks. Build inputs must be generated separately from contributors’ own game copies. The exact runtime capture corpus is also local-only. See the contributor handoff for the dependency and input inventory.

Outstanding validation

No game was launched for publication. The last captured hub/LOTR images still contained blank portraits and rendering artifacts. The current gradient/bias combination has not been visually validated. Portraits/button prompts, actual internal 1920×1080 rendering, stable 60 FPS, three story levels/cutscenes and AMD hardware validation remain outstanding. The depth-bias helper tests establish decoding behavior, not a visual repair. Game-derived shader banks, generated code, captures, saves, and Avengers reference payloads are not published.