Add OpenRouter AI texture generation mode to the asset pipeline

- tools/generate_textures.py: --ai mode (OPENROUTER_API_KEY, --prompt,
  --name, --model, --size; base64 PNG decode; clamp to <=1024px) with a
  default batch covering the authored texture set; seedream minimum-size
  handled by requesting 2048 and downscaling
- Generated AI asset set committed under assets/textures/ai/ (15 files,
  1024x1024 each), kept separate from the deterministic PIL set
- README/MEMORY/TODO document the mode and live verification
- (work authored in a parallel session; linted/verified here — 57/57 tests)

TODO.md

@@ -1,5 +1,15 @@
 # TODO
 
+## 🎨 AI texture generation (OpenRouter)
+
+- [x] Extend `tools/generate_textures.py` with an OpenRouter image-API mode
+      (`--ai`, `--prompt`, `--name`, `--model`, `--size`), decoding base64 PNG
+      responses and clamping output to ≤1024px.
+- [x] Verify a live `--ai` generation against a real `OPENROUTER_API_KEY`
+      (generated `ai/brick_facade.png`, `ai/concrete.png`, `ai/asphalt.png` at
+      1024×1024; fixed the seedream min-pixel requirement by requesting 2048px
+      and downscaling).
+
 ## 🪟 Windows installer (self-contained, all game data)
 
 Goal: a real Windows installer that includes the entire game (executable + data),