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/Containerfile b/Containerfile
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+# Containerfile for Forts Clone build environment
+# Builds all C/C++ dependencies from source, installs to /usr/local.
+# Usage:
+#   podman build -t forts-clone-dev .
+#   podman run -it --rm -v $(pwd):/src:Z --userns=keep-id forts-clone-dev
+
+FROM docker.io/library/fedora:41
+
+# =============================================================================
+# All system packages in one layer
+# =============================================================================
+RUN dnf -y update && dnf -y install \
+    gcc-c++ clang lld meson ninja-build cmake git gdb valgrind \
+    SDL3-devel lua-devel glm-devel \
+    mesa-libGL-devel mesa-libEGL-devel \
+    libX11-devel libXcursor-devel libXi-devel libXinerama-devel libXrandr-devel \
+    wayland-devel libdrm-devel libxkbcommon-devel libxkbcommon-x11-devel \
+    vulkan-loader-devel \
+    pkg-config python3 which \
+    && dnf clean all
+
+# =============================================================================
+# bgfx -- cross-platform GPU rendering
+# =============================================================================
+WORKDIR /tmp/bgfx_build
+RUN git clone --recursive --depth 1 https://github.com/bkaradzic/bgfx.cmake.git . \
+    && cmake -B build -G Ninja \
+        -DCMAKE_BUILD_TYPE=Release \
+        -DCMAKE_INSTALL_PREFIX=/usr/local \
+        -DBGFX_BUILD_TOOLS=OFF \
+        -DBGFX_BUILD_EXAMPLES=OFF \
+        -DBGFX_BUILD_TESTS=OFF \
+    && cmake --build build \
+    && cmake --install build \
+    && rm -rf /tmp/bgfx_build
+
+# =============================================================================
+# Box2D v3 -- deterministic 2D physics engine
+# Built directly from source to avoid CMake FetchContent issues.
+# =============================================================================
+WORKDIR /tmp/box2d_build
+RUN git clone --depth 1 --branch v3.1.1 https://github.com/erincatto/box2d.git . \
+    && mkdir -p /usr/local/include/box2d \
+    && cp -r include/box2d/*.h /usr/local/include/box2d/ \
+    && cp -r include/box2d/*.h /usr/local/include/ \
+    && gcc -c src/*.c -I include -I src -O2 -DBOX2D_ENABLE_ASSERT=0 \
+    && ar rcs libbox2d.a *.o \
+    && cp libbox2d.a /usr/local/lib/ \
+    && rm -rf /tmp/box2d_build
+
+# =============================================================================
+# SoLoud -- game audio engine
+# Built directly from source (contrib CMake is fragile).
+# =============================================================================
+RUN dnf -y install alsa-lib-devel pulseaudio-libs-devel && dnf clean all
+WORKDIR /tmp/soloud_build
+RUN git clone --depth 1 https://github.com/jarikomppa/soloud.git . \
+    && mkdir -p /usr/local/include/soloud \
+    && cp include/*.h /usr/local/include/soloud/ \
+    && g++ -c src/core/*.cpp src/audiosource/*/*.cpp src/filter/*.cpp \
+        src/backend/alsa/*.cpp src/c_api/*.cpp \
+        -I include -O2 -fPIC -DWITH_ALSA \
+    && ar rcs libsoloud.a *.o \
+    && cp libsoloud.a /usr/local/lib/ \
+    && rm -rf /tmp/soloud_build
+
+# =============================================================================
+# ENet -- UDP networking library
+# =============================================================================
+WORKDIR /tmp/enet_build
+RUN git clone --depth 1 --branch v1.3.18 https://github.com/lsalzman/enet.git . \
+    && cmake -B build -G Ninja \
+        -DCMAKE_BUILD_TYPE=Release \
+        -DCMAKE_INSTALL_PREFIX=/usr/local \
+    && cmake --build build \
+    && cmake --install build \
+    && rm -rf /tmp/enet_build
+
+# =============================================================================
+# =============================================================================
+# sol2 -- Lua C++ bindings (header-only)
+# =============================================================================
+WORKDIR /tmp/sol2_build
+RUN git clone --depth 1 --branch v3.5.0 https://github.com/ThePhD/sol2.git . \
+    && cp -r include/sol /usr/local/include/ \
+    && rm -rf /tmp/sol2_build
+
+# =============================================================================
+# EnTT -- ECS library (header-only)
+# =============================================================================
+WORKDIR /tmp/entt_build
+RUN git clone --depth 1 --branch v3.14.0 https://github.com/skypjack/entt.git . \
+    && cp -r src/entt /usr/local/include/ \
+    && rm -rf /tmp/entt_build
+
+# =============================================================================
+# Lua 5.4 -- built as a STATIC archive (liblua.a) so the game binary carries
+# Lua itself and needs no liblua .so at runtime. Fedora's shared soname
+# (liblua-5.4.so) differs from other distros (liblua5.4.so), which breaks
+# running the container-built binary on the host. Appended LAST so none of the
+# cached dependency layers above have to rebuild.
+# =============================================================================
+WORKDIR /tmp/lua_build
+RUN python3 -c "import urllib.request; urllib.request.urlretrieve('https://www.lua.org/ftp/lua-5.4.7.tar.gz','lua.tar.gz')" \
+    && tar xf lua.tar.gz \
+    && cd lua-5.4.7/src \
+    && gcc -O2 -fPIC -DLUA_USE_LINUX -c $(ls *.c | grep -vE '^(lua|luac)\.c$') \
+    && ar rcs liblua.a *.o \
+    && install -m644 liblua.a /usr/local/lib/ \
+    && install -m644 lua.h luaconf.h lualib.h lauxlib.h /usr/local/include/ \
+    && printf 'extern "C" {\n#include "lua.h"\n#include "lualib.h"\n#include "lauxlib.h"\n}\n' \
+        > /usr/local/include/lua.hpp \
+    && rm -rf /tmp/lua_build
+
+# =============================================================================
+# Ready
+# =============================================================================
+WORKDIR /src
+CMD ["bash"]