feat(branch): phyllotactic spiral and upward angle

Default rotation to the golden angle (137.5°) so successive branches
spiral naturally around the stem. Add an `angle` input (-90..90°) that
tilts branches between horizontal and along-stem before rotating around
the stem axis.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

nodes/max/plantarium/branch/src/input.json

@@ -83,6 +83,14 @@
       "min": 0,
       "max": 360,
       "step": 0.01,
+      "value": 137.5
+    },
+    "angle": {
+      "type": "float",
+      "description": "Upward tilt of branches. 0 = horizontal, positive = upward, negative = drooping.",
+      "min": -90,
+      "max": 90,
+      "step": 1,
       "value": 0
     }
   }

nodes/max/plantarium/branch/src/lib.rs

@@ -78,7 +78,9 @@ pub fn execute(input: &[i32]) -> Vec<i32> {
                 continue;
             }
 
-            let branch_direction = rotate_vector_by_angle(orthogonal, direction, rotation_angle);
+            let up_angle = evaluate_float(args[10]) * PI / 180.0;
+            let tilted = (orthogonal * up_angle.cos() + direction * up_angle.sin()).normalize();
+            let branch_direction = rotate_vector_by_angle(tilted, direction, rotation_angle);
 
             log!(
                 "BRANCH depth: {}, branch_origin: {:?}, direction_at: {:?}, branch_direction: {:?}",