feat: plumb spin parameter into GPU uniform

assets/shaders/black_hole.wgsl

@@ -33,7 +33,7 @@ struct BlackHoleUniforms {
     grid_enabled: u32,
     planet_count: u32,
     steps: u32,
-    _pad4: f32,
+    spin: f32,
     _pad5: f32,
 };
 

src/render/material.rs

@@ -37,7 +37,7 @@ pub struct BlackHoleUniforms {
     pub grid_enabled: u32,
     pub planet_count: u32,
     pub steps: u32,
-    pub _pad4: f32,
+    pub spin: f32,       // Phase 2: dimensionless Kerr spin χ = a/M ∈ [0,1].
     pub _pad5: f32,
 }
 
@@ -69,7 +69,7 @@ impl Default for BlackHoleUniforms {
             grid_enabled: 0,
             planet_count: 0,
             steps: 300,
-            _pad4: 0.0,
+            spin: 0.0,
             _pad5: 0.0,
         }
     }

src/render/plugin.rs

@@ -226,5 +226,6 @@ fn mirror_params(
         u.grid_enabled = params.grid_enabled as u32;
         u.planet_count = params.planet_count;
         u.steps = params.steps;
+        u.spin = params.spin;
     }
 }