Add normal mapping and parallax occlusion mapping for WMO surfaces

Generate normal+height maps from diffuse textures at load time using
luminance-to-height and Sobel 3x3 filtering. Compute per-vertex tangents
via Lengyel's method for TBN basis construction.

Fragment shader uses screen-space UV derivatives (dFdx/dFdy) for smooth
LOD crossfade and angle-adaptive POM sample counts. Flat textures
naturally produce low height variance, causing POM to self-select off.

Settings: Normal Mapping on by default, POM off by default with
Low/Medium/High quality presets. Persisted across sessions.

include/ui/game_screen.hpp

@@ -103,6 +103,9 @@ private:
     bool pendingUseOriginalSoundtrack = true;
     int pendingGroundClutterDensity = 100;
     int pendingAntiAliasing = 0;  // 0=Off, 1=2x, 2=4x, 3=8x
+    bool pendingNormalMapping = true;   // on by default
+    bool pendingPOM = false;            // off by default (expensive)
+    int pendingPOMQuality = 1;          // 0=Low(16), 1=Medium(32), 2=High(64)
 
     // UI element transparency (0.0 = fully transparent, 1.0 = fully opaque)
     float uiOpacity_ = 0.65f;
@@ -112,6 +115,7 @@ private:
     bool minimapSettingsApplied_ = false;
     bool volumeSettingsApplied_ = false;  // True once saved volume settings applied to audio managers
     bool msaaSettingsApplied_ = false;   // True once saved MSAA setting applied to renderer
+    bool normalMapSettingsApplied_ = false;  // True once saved normal map/POM settings applied
 
     // Mute state: mute bypasses master volume without touching slider values
     bool soundMuted_ = false;