Add NoiseMedium for mixing materials based on noise patterns

Create a new NoiseMedium class that uses noise patterns to mix between
two materials, similar to how CheckeredYPlane works. This provides more
versatile material combinations based on Perlin, FBM, Worley, and other
noise patterns.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: peterbraden

demo/scenes/noise-medium-test.json

@@ -30,7 +30,8 @@
     },
     "GLASS": {
       "type": "dielectric",
-      "ref_idx": 1.5
+      "refractive_index": 1.5,
+      "attenuate": [0.95, 0.95, 0.95]
     },
     "WHITE_MARBLE": {
       "type": "lambertian",

src/scenefile.rs

@@ -404,8 +404,14 @@ impl SceneFile {
     }
 
     pub fn parse_medium_ref(key: &Value, materials: &Map<String, Value>, media: &Map<String, Value> ) -> Option<Box<dyn Medium + Sync + Send>> {
-        let props = media.get(&SceneFile::parse_string(key)).unwrap();
-        return SceneFile::parse_medium(props, materials);
+        let medium_name = SceneFile::parse_string(key);
+        match media.get(&medium_name) {
+            Some(props) => SceneFile::parse_medium(props, materials),
+            None => {
+                eprintln!("ERROR: Medium '{}' not found in media map. This is a fatal error.", medium_name);
+                panic!("Medium '{}' not found in media map", medium_name);
+            }
+        }
     }
 
     pub fn parse_medium(o: &Value, materials: &Map<String, Value>) -> Option<Box<dyn Medium + Sync + Send>> {
@@ -513,8 +519,23 @@ impl SceneFile {
 
         if t == "noise_medium" {
             // Get the two materials to mix between
-            let m1 = SceneFile::parse_material_ref(&o["m1"], materials).unwrap();
-            let m2 = SceneFile::parse_material_ref(&o["m2"], materials).unwrap();
+            let m1_name = SceneFile::parse_string(&o["m1"]);
+            let m1 = match SceneFile::parse_material_ref(&o["m1"], materials) {
+                Some(mat) => mat,
+                None => {
+                    eprintln!("ERROR: Material '{}' not found for NoiseMedium m1. This is a fatal error.", m1_name);
+                    panic!("Material '{}' for NoiseMedium m1 not found", m1_name);
+                }
+            };
+            
+            let m2_name = SceneFile::parse_string(&o["m2"]);
+            let m2 = match SceneFile::parse_material_ref(&o["m2"], materials) {
+                Some(mat) => mat,
+                None => {
+                    eprintln!("ERROR: Material '{}' not found for NoiseMedium m2. This is a fatal error.", m2_name);
+                    panic!("Material '{}' for NoiseMedium m2 not found", m2_name);
+                }
+            };
 
             // Parse the threshold value (default to 0.5)
             let threshold = SceneFile::parse_number(&o["threshold"], 0.5);