feat: seeded RNG with per-label deterministic rolls

src/sim/rng.js

@@ -0,0 +1,28 @@
+// FNV-1a 32-bit string hash.
+export function hash32(str) {
+  let h = 0x811c9dc5;
+  for (let i = 0; i < str.length; i++) {
+    h ^= str.charCodeAt(i);
+    h = Math.imul(h, 0x01000193);
+  }
+  return h >>> 0;
+}
+
+// mulberry32 PRNG: returns function yielding uniforms in [0,1).
+export function makeRng(seed) {
+  let a = seed >>> 0;
+  return function () {
+    a |= 0;
+    a = (a + 0x6d2b79f5) | 0;
+    let t = Math.imul(a ^ (a >>> 15), 1 | a);
+    t = (t + Math.imul(t ^ (t >>> 7), 61 | t)) ^ t;
+    return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
+  };
+}
+
+// One deterministic uniform in [0,1) for a (seed, label) pair.
+// This is the whole hidden-variable mechanism: outcomes are fixed
+// the moment their label exists, revealed whenever first asked for.
+export function rollFor(seed, label) {
+  return makeRng(hash32(seed + ':' + label))();
+}