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, 70 insertions, 0 deletions

diff --git a/src/engine/physics.cpp b/src/engine/physics.cpp
new file mode 100644
index 0000000..af5d8ee
--- /dev/null
+++ b/src/engine/physics.cpp
@@ -0,0 +1,70 @@
+#include "engine/physics.hpp"
+#include <cstdio>
+
+PhysicsWorld::PhysicsWorld() {
+    b2WorldDef def = b2DefaultWorldDef();
+    def.gravity = {0.0f, -10.0f};
+    world_id_ = b2CreateWorld(&def);
+    printf("Physics: Box2D world created\n");
+}
+
+PhysicsWorld::~PhysicsWorld() {
+    b2DestroyWorld(world_id_);
+}
+
+void PhysicsWorld::step(float dt) {
+    // High sub-step count for stable distance joint networks
+    b2World_Step(world_id_, dt, 8);
+}
+
+PhysicsBody* PhysicsWorld::add_static_box(b2Vec2 center,
+                                           float half_w, float half_h,
+                                           float r, float g, float b) {
+    b2BodyDef body_def = b2DefaultBodyDef();
+    body_def.type     = b2_staticBody;
+    body_def.position = center;
+
+    b2BodyId body_id = b2CreateBody(world_id_, &body_def);
+
+    b2ShapeDef shape_def = b2DefaultShapeDef();
+    b2Polygon  box       = b2MakeBox(half_w, half_h);
+    b2CreatePolygonShape(body_id, &shape_def, &box);
+
+    PhysicsBody pb;
+    pb.id        = body_id;
+    pb.size      = {half_w, half_h};
+    pb.red       = r;
+    pb.green     = g;
+    pb.blue      = b;
+    pb.alpha     = 1.0f;
+    pb.is_static = true;
+    bodies_.push_back(pb);
+    return &bodies_.back();
+}
+
+PhysicsBody* PhysicsWorld::add_dynamic_box(b2Vec2 center,
+                                            float half_w, float half_h,
+                                            float r, float g, float b) {
+    b2BodyDef body_def = b2DefaultBodyDef();
+    body_def.type     = b2_dynamicBody;
+    body_def.position = center;
+
+    b2BodyId body_id = b2CreateBody(world_id_, &body_def);
+
+    b2ShapeDef shape_def = b2DefaultShapeDef();
+    shape_def.density = 1.0f;
+    shape_def.material.friction = 0.3f;
+    b2Polygon box = b2MakeBox(half_w, half_h);
+    b2CreatePolygonShape(body_id, &shape_def, &box);
+
+    PhysicsBody pb;
+    pb.id        = body_id;
+    pb.size      = {half_w, half_h};
+    pb.red       = r;
+    pb.green     = g;
+    pb.blue      = b;
+    pb.alpha     = 1.0f;
+    pb.is_static = false;
+    bodies_.push_back(pb);
+    return &bodies_.back();
+}