feat: seeded RNG with per-label deterministic rolls

tests/rng.test.js

@@ -0,0 +1,32 @@
+import { describe, it, expect } from 'vitest';
+import { hash32, makeRng, rollFor } from '../src/sim/rng.js';
+
+describe('rng', () => {
+  it('hash32 is deterministic and spreads', () => {
+    expect(hash32('abc')).toBe(hash32('abc'));
+    expect(hash32('abc')).not.toBe(hash32('abd'));
+  });
+
+  it('makeRng produces a deterministic sequence in [0,1)', () => {
+    const a = makeRng(42), b = makeRng(42);
+    for (let i = 0; i < 100; i++) {
+      const v = a();
+      expect(v).toBe(b());
+      expect(v).toBeGreaterThanOrEqual(0);
+      expect(v).toBeLessThan(1);
+    }
+  });
+
+  it('rollFor depends on seed and label, independently', () => {
+    expect(rollFor('s1', 'x')).toBe(rollFor('s1', 'x'));
+    expect(rollFor('s1', 'x')).not.toBe(rollFor('s1', 'y'));
+    expect(rollFor('s1', 'x')).not.toBe(rollFor('s2', 'x'));
+  });
+
+  it('rollFor is roughly uniform', () => {
+    let sum = 0;
+    for (let i = 0; i < 2000; i++) sum += rollFor('u', `l${i}`);
+    expect(sum / 2000).toBeGreaterThan(0.45);
+    expect(sum / 2000).toBeLessThan(0.55);
+  });
+});