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/data/textures/CREDITS.md b/data/textures/CREDITS.md
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+# Placeholder texture credits
+
+All placeholder art here is **CC0 1.0 (public domain)** by **Kenney** — https://kenney.nl
+(no attribution required; credited anyway). Pulled from the mirror
+https://github.com/iwenzhou/kenney.
+
+| File | Source pack | Original |
+|------|-------------|----------|
+| wood.png | Platformer pack (Base) | Tiles/box.png |
+| bgbrace.png | Platformer pack (Base) | Tiles/boxAlt.png |
+| ground.png | Platformer pack (Base) | Tiles/dirtCenter.png |
+| weapon_gun.png | Space Shooter (300) | Parts/gun07.png |
+| weapon_turret.png | Space Shooter (300) | Parts/turretBase_big.png |
+| projectile_laser.png | Space Shooter (300) | Lasers/laserBlue01.png |
+
+These are temporary placeholders. Replace with original art before release.
+
+Third-party code: `third_party/stb/stb_image*.h` — public domain (stb, Sean Barrett).
+
+## M5 devices (added)
+| reactor_player.png | Platformer (Base) | Items/gemGreen.png |
+| reactor_enemy.png  | Platformer (Base) | Items/gemRed.png    |
+| deposit.png        | Platformer (Base) | Items/gemYellow.png |
diff --git a/data/textures/bgbrace.png b/data/textures/bgbrace.png
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diff --git a/data/textures/deposit.png b/data/textures/deposit.png
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+++ b/data/textures/deposit.png
diff --git a/data/textures/ground.png b/data/textures/ground.png
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+++ b/data/textures/ground.png
diff --git a/data/textures/projectile_laser.png b/data/textures/projectile_laser.png
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diff --git a/data/textures/reactor_enemy.png b/data/textures/reactor_enemy.png
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+++ b/data/textures/reactor_enemy.png
diff --git a/data/textures/reactor_player.png b/data/textures/reactor_player.png
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+++ b/data/textures/reactor_player.png
diff --git a/data/textures/weapon_gun.png b/data/textures/weapon_gun.png
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+++ b/data/textures/weapon_gun.png
diff --git a/data/textures/weapon_turret.png b/data/textures/weapon_turret.png
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+++ b/data/textures/weapon_turret.png
diff --git a/data/textures/wood.png b/data/textures/wood.png
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+++ b/data/textures/wood.png