Refactor random number generation to factory

- Use a `make-random` factory to create isolated random number
  generators.
- Remove the global `prng` from the `Environment`.
- Ensure `make-random` can be called with or without a seed.

src/integration_test.rs

@@ -175,12 +175,13 @@ mod tests {
         let env1 = Environment::new();
         let env2 = Environment::new();
 
-        // 1. Seed env1
-        env1.run_script("(seed! 123)").unwrap();
-        let val1_a = env1.run_script("(random)").unwrap();
+        // 1. Create a seeded generator in env1
+        env1.run_script("(def rand (make-random 123))").unwrap();
+        let val1_a = env1.run_script("(rand)").unwrap();
 
-        // 2. env2 should have its own default seed state
-        let val2_a = env2.run_script("(random)").unwrap();
+        // 2. env2 should have its own default seed state for its generators
+        env2.run_script("(def rand (make-random))").unwrap();
+        let val2_a = env2.run_script("(rand)").unwrap();
 
         // They are highly unlikely to be equal by default,
         // and seeding env1 MUST not have seeded env2.
@@ -189,9 +190,9 @@ mod tests {
             "Environments must have isolated PRNG states"
         );
 
-        // 3. Seed env2 differently
-        env2.run_script("(seed! 123)").unwrap();
-        let val2_b = env2.run_script("(random)").unwrap();
+        // 3. Create another generator in env2 with the same seed
+        env2.run_script("(def rand-same (make-random 123))").unwrap();
+        let val2_b = env2.run_script("(rand-same)").unwrap();
 
         // After same seeding, they should match (isolated but identical seed)
         assert_eq!(
@@ -205,12 +206,12 @@ mod tests {
         let env = Environment::new();
 
         // 1. First run with seed 42
-        env.run_script("(seed! 42)").unwrap();
-        let val1 = env.run_script("(random)").unwrap();
+        env.run_script("(def rand1 (make-random 42))").unwrap();
+        let val1 = env.run_script("(rand1)").unwrap();
 
         // 2. Second run with same seed 42
-        env.run_script("(seed! 42)").unwrap();
-        let val2 = env.run_script("(random)").unwrap();
+        env.run_script("(def rand2 (make-random 42))").unwrap();
+        let val2 = env.run_script("(rand2)").unwrap();
 
         assert_eq!(
             val1, val2,
@@ -218,8 +219,8 @@ mod tests {
         );
 
         // 3. Third run with different seed
-        env.run_script("(seed! 123)").unwrap();
-        let val3 = env.run_script("(random)").unwrap();
+        env.run_script("(def rand3 (make-random 123))").unwrap();
+        let val3 = env.run_script("(rand3)").unwrap();
         assert_ne!(val1, val3, "Random results must differ for different seeds");
     }