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.

1 files changed, 41 insertions, 0 deletions

diff --git a/src/engine/physics.hpp b/src/engine/physics.hpp
new file mode 100644
index 0000000..6a8abda
--- /dev/null
+++ b/src/engine/physics.hpp
@@ -0,0 +1,41 @@
+#pragma once
+
+#include <box2d/box2d.h>
+#include <vector>
+
+// Box2D rigid-body world. The building structure runs on the custom Verlet
+// solver (see build_graph / verlet); this exists as the foundation for M4
+// projectiles and any terrain collision they need.
+struct PhysicsBody {
+    b2BodyId  id;
+    b2Vec2    size;      // half-width, half-height for box shapes
+    float     red, green, blue, alpha;
+    bool      is_static;
+};
+
+class PhysicsWorld {
+public:
+    PhysicsWorld();
+    ~PhysicsWorld();
+
+    PhysicsWorld(const PhysicsWorld&) = delete;
+    PhysicsWorld& operator=(const PhysicsWorld&) = delete;
+
+    void step(float dt);
+
+    // Create a static box (terrain, walls).
+    PhysicsBody* add_static_box(b2Vec2 center, float half_w, float half_h,
+                                float r = 0.3f, float g = 0.3f, float b = 0.3f);
+
+    // Create a dynamic box (projectiles, debris).
+    PhysicsBody* add_dynamic_box(b2Vec2 center, float half_w, float half_h,
+                                 float r = 0.8f, float g = 0.6f, float b = 0.2f);
+
+    const std::vector<PhysicsBody>& bodies() const { return bodies_; }
+
+    b2WorldId world_id() const { return world_id_; }
+
+private:
+    b2WorldId               world_id_;
+    std::vector<PhysicsBody> bodies_;
+};