Initial import: LibreForts (M0-M5 in progress)

Open-source Forts clone: custom C++20 engine + data-driven (Lua) 2D physics
artillery RTS.

- Building: node/strut graph on a stiff mass-spring solver (canon Forts model),
  triangulation rigidity, axial + 30-degree angle-stress breaking, cascading
  collapse, fire (DoT + spread), ground destroys debris.
- Weapons (M4): cannon (ballistic) + laser (beam/ignite) with select-and-aim-in-
  arc UX; splash / beam damage; bg-brace passthrough.
- Devices + economy (M5): reactors + win/loss + restart, mines/turbines/battery,
  metal deposits, storage caps, per-shot energy cost.
- Data-driven via Lua/sol2 with a layered mod loader; enemy-fort scenario mod.
- Renderer: SDL3 + textures + ImGui dev UI. stb_image + ImGui vendored.
- Assets: CC0 placeholders only; real game art loaded at runtime from the user's
  own install via forts: paths (bring-your-own; nothing copyrighted committed).

See README.md / CLAUDE.md and research/ + requirements/ for detail.

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diff --git a/third_party/imgui/backends/imgui_impl_sdlrenderer3.h b/third_party/imgui/backends/imgui_impl_sdlrenderer3.h
new file mode 100644
index 0000000..7d4c609
--- /dev/null
+++ b/third_party/imgui/backends/imgui_impl_sdlrenderer3.h
@@ -0,0 +1,51 @@
+// dear imgui: Renderer Backend for SDL_Renderer for SDL3
+// (Requires: SDL 3.0.0+)
+
+// (**IMPORTANT: SDL 3.0.0 is NOT YET RELEASED AND CURRENTLY HAS A FAST CHANGING API. THIS CODE BREAKS OFTEN AS SDL3 CHANGES.**)
+
+// Note how SDL_Renderer is an _optional_ component of SDL3.
+// For a multi-platform app consider using e.g. SDL+DirectX on Windows and SDL+OpenGL on Linux/OSX.
+// If your application will want to render any non trivial amount of graphics other than UI,
+// please be aware that SDL_Renderer currently offers a limited graphic API to the end-user and
+// it might be difficult to step out of those boundaries.
+
+// Implemented features:
+//  [X] Renderer: User texture binding. Use 'SDL_Texture*' as ImTextureID. Read the FAQ about ImTextureID!
+//  [X] Renderer: Large meshes support (64k+ vertices) with 16-bit indices.
+//  [X] Renderer: Expose selected render state for draw callbacks to use. Access in '(ImGui_ImplXXXX_RenderState*)GetPlatformIO().Renderer_RenderState'.
+
+// You can copy and use unmodified imgui_impl_* files in your project. See examples/ folder for examples of using this.
+// Prefer including the entire imgui/ repository into your project (either as a copy or as a submodule), and only build the backends you need.
+// Learn about Dear ImGui:
+// - FAQ                  https://dearimgui.com/faq
+// - Getting Started      https://dearimgui.com/getting-started
+// - Documentation        https://dearimgui.com/docs (same as your local docs/ folder).
+// - Introduction, links and more at the top of imgui.cpp
+
+#pragma once
+#include "imgui.h"      // IMGUI_IMPL_API
+#ifndef IMGUI_DISABLE
+
+struct SDL_Renderer;
+
+// Follow "Getting Started" link and check examples/ folder to learn about using backends!
+IMGUI_IMPL_API bool     ImGui_ImplSDLRenderer3_Init(SDL_Renderer* renderer);
+IMGUI_IMPL_API void     ImGui_ImplSDLRenderer3_Shutdown();
+IMGUI_IMPL_API void     ImGui_ImplSDLRenderer3_NewFrame();
+IMGUI_IMPL_API void     ImGui_ImplSDLRenderer3_RenderDrawData(ImDrawData* draw_data, SDL_Renderer* renderer);
+
+// Called by Init/NewFrame/Shutdown
+IMGUI_IMPL_API bool     ImGui_ImplSDLRenderer3_CreateFontsTexture();
+IMGUI_IMPL_API void     ImGui_ImplSDLRenderer3_DestroyFontsTexture();
+IMGUI_IMPL_API bool     ImGui_ImplSDLRenderer3_CreateDeviceObjects();
+IMGUI_IMPL_API void     ImGui_ImplSDLRenderer3_DestroyDeviceObjects();
+
+// [BETA] Selected render state data shared with callbacks.
+// This is temporarily stored in GetPlatformIO().Renderer_RenderState during the ImGui_ImplSDLRenderer3_RenderDrawData() call.
+// (Please open an issue if you feel you need access to more data)
+struct ImGui_ImplSDLRenderer3_RenderState
+{
+    SDL_Renderer*       Renderer;
+};
+
+#endif // #ifndef IMGUI_DISABLE