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)

README.md

@@ -136,7 +136,7 @@ The architecture maps onto Godot 4 nodes, scenes, and custom resources as descri
 - **Navigation:** `NavigationRegion3D` and `NavigationAgent3D`
 - **NPC behavior:** Custom lightweight finite state machines driven by `NpcProfile` resources
 - **UI and audio:** Godot `Control` nodes, themes, audio buses, and `AudioStreamPlayer` nodes
-- **Art assets:** authored PNGs in `assets/textures/` (≤1024px, power-of-two) produced by `tools/generate_textures.py` and imported by Godot; structural geometry stays procedural (`WorldBuilder`)
+- **Art assets:** authored PNGs in `assets/textures/` (≤1024px, power-of-two) produced by `tools/generate_textures.py` and imported by Godot; structural geometry stays procedural (`WorldBuilder`). The same script can generate textures through the OpenRouter image API (`--ai`, requires `OPENROUTER_API_KEY`).
 - **Testing:** GUT (Godot Unit Test) in headless mode
 - **Source control:** git on a Gitea host with Gitea Actions (`.gitea/workflows/build.yml`) for headless tests and desktop release builds
 - **Targets:** Windows, Linux, and macOS desktop first; web later