diff --git a/index.ts b/index.ts
deleted file mode 100644
index f67b2c6..0000000
--- a/index.ts
+++ /dev/null
@@ -1 +0,0 @@
-console.log("Hello via Bun!");
\ No newline at end of file
diff --git a/package.json b/package.json
index f4cd02e..b3ccbad 100644
--- a/package.json
+++ b/package.json
@@ -1,8 +1,13 @@
 {
   "name": "stsheets",
-  "module": "index.ts",
   "type": "module",
   "private": true,
+  "scripts": {
+    "test": "bun test",
+    "typecheck": "bunx tsc --noEmit",
+    "fetch-models": "bun scripts/fetch-models.ts",
+    "bench": "bun src/bench/cli.ts"
+  },
   "devDependencies": {
     "@types/bun": "latest"
   },
diff --git a/scripts/fetch-models.ts b/scripts/fetch-models.ts
new file mode 100644
index 0000000..cff231b
--- /dev/null
+++ b/scripts/fetch-models.ts
@@ -0,0 +1,60 @@
+// Downloads model files into models/ and verifies their sha256.
+// Files already present with a matching hash are skipped.
+import { join } from "node:path";
+
+interface ModelFile {
+  path: string; // relative to repo root
+  url: string;
+  sha256: string;
+}
+
+export const MODELS: ModelFile[] = [
+  {
+    path: "models/demucs/htdemucs_6s.onnx",
+    url: "https://huggingface.co/Kani95/htdemucs-6s-ort/resolve/main/htdemucs_6s.onnx",
+    sha256: "35cb61d4d35173ff1d16df7393f41f05651ffdfe13d3c10e06abc26708bd6933",
+  },
+  {
+    path: "models/basic-pitch/nmp.onnx",
+    url: "https://github.com/spotify/basic-pitch/raw/main/basic_pitch/saved_models/icassp_2022/nmp.onnx",
+    sha256: "2c3c1d144bfa61ad236e92e169c13535c880469a12a047d4e73451f2c059a0ec",
+  },
+];
+
+const ROOT = join(import.meta.dir, "..");
+
+async function sha256Of(file: string): Promise<string> {
+  const hasher = new Bun.CryptoHasher("sha256");
+  hasher.update(new Uint8Array(await Bun.file(file).arrayBuffer()));
+  return hasher.digest("hex");
+}
+
+export async function fetchModels(root = ROOT): Promise<void> {
+  for (const m of MODELS) {
+    const dest = join(root, m.path);
+    if (await Bun.file(dest).exists()) {
+      const have = await sha256Of(dest);
+      if (have === m.sha256) {
+        console.log(`ok       ${m.path}`);
+        continue;
+      }
+      console.log(`mismatch ${m.path} (have ${have}), re-downloading`);
+    }
+    console.log(`fetch    ${m.url}`);
+    const res = await fetch(m.url, { redirect: "follow" });
+    if (!res.ok) throw new Error(`download failed for ${m.url}: HTTP ${res.status}`);
+    const bytes = new Uint8Array(await res.arrayBuffer());
+    const hasher = new Bun.CryptoHasher("sha256");
+    hasher.update(bytes);
+    const got = hasher.digest("hex");
+    if (got !== m.sha256) {
+      throw new Error(`sha256 mismatch for ${m.path}: expected ${m.sha256}, got ${got}`);
+    }
+    await Bun.write(dest, bytes);
+    console.log(`saved    ${m.path}`);
+  }
+}
+
+if (import.meta.main) {
+  await fetchModels();
+}
diff --git a/src/core/rng.ts b/src/core/rng.ts
new file mode 100644
index 0000000..2e597bc
--- /dev/null
+++ b/src/core/rng.ts
@@ -0,0 +1,33 @@
+export interface Rng {
+  /** Uniform in [0, 1). */
+  next(): number;
+  /** Uniform integer in [lo, hi], inclusive. */
+  int(lo: number, hi: number): number;
+  pick<T>(a: readonly T[]): T;
+  chance(p: number): boolean;
+}
+
+// mulberry32
+export function createRng(seed: number): Rng {
+  let s = seed >>> 0;
+  const next = (): number => {
+    s = (s + 0x6d2b79f5) >>> 0;
+    let t = s;
+    t = Math.imul(t ^ (t >>> 15), t | 1);
+    t ^= t + Math.imul(t ^ (t >>> 7), t | 61);
+    return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
+  };
+  return {
+    next,
+    int(lo, hi) {
+      return lo + Math.floor(next() * (hi - lo + 1));
+    },
+    pick<T>(a: readonly T[]): T {
+      if (a.length === 0) throw new Error("rng.pick: empty array");
+      return a[Math.floor(next() * a.length)] as T;
+    },
+    chance(p) {
+      return next() < p;
+    },
+  };
+}
diff --git a/src/core/time.ts b/src/core/time.ts
new file mode 100644
index 0000000..e1433a5
--- /dev/null
+++ b/src/core/time.ts
@@ -0,0 +1,10 @@
+import { TPQ } from "./types.ts";
+
+// One quarter note = TPQ ticks.
+export function ticksToSeconds(tick: number, bpm: number): number {
+  return (tick / TPQ) * (60 / bpm);
+}
+
+export function secondsToTicks(s: number, bpm: number): number {
+  return s * (bpm / 60) * TPQ;
+}
diff --git a/src/core/types.ts b/src/core/types.ts
new file mode 100644
index 0000000..205e244
--- /dev/null
+++ b/src/core/types.ts
@@ -0,0 +1,13 @@
+export type Role = "guitar" | "bass" | "keys" | "vocals" | "drums";
+export type FxPreset = "none" | "clean" | "clean-chorus" | "light-drive";
+// string numbering: 1 = highest-pitched string (alphaTex convention). Drums: pitch = GM drum number.
+export interface ScoreNote { tick: number; dur: number; pitch: number; string?: number; fret?: number }
+export interface Track { name: string; role: Role; program: number; tuning?: number[]; pan: number; fx: FxPreset; gainDb: number; notes: ScoreNote[] }
+export interface Song { title: string; tier: string; seed: number; bpm: number; timeSig: [number, number]; bars: number; key: string; tracks: Track[] }
+export interface TimedNote { onset: number; offset: number; pitch: number; velocity?: number; string?: number; fret?: number }
+export interface TruthTrack { name: string; role: Role; tuning?: number[]; notes: TimedNote[] }
+export interface Grid { bpm: number; firstDownbeat: number; beatsPerBar: number }
+export interface Audio { sampleRate: number; channels: Float32Array[] }
+export const TPQ = 960;
+export const GUITAR_STANDARD = [64, 59, 55, 50, 45, 40];
+export const BASS_STANDARD = [43, 38, 33, 28];
diff --git a/tests/core/rng.test.ts b/tests/core/rng.test.ts
new file mode 100644
index 0000000..2e1c523
--- /dev/null
+++ b/tests/core/rng.test.ts
@@ -0,0 +1,10 @@
+import { test, expect } from "bun:test";
+import { createRng } from "../../src/core/rng.ts";
+
+test("rng deterministic", () => {
+  const a = createRng(42), b = createRng(42), c = createRng(43);
+  const sa = [a.next(), a.next(), a.next()];
+  expect([b.next(), b.next(), b.next()]).toEqual(sa);
+  expect(c.next()).not.toBe(sa[0]);
+  const r = createRng(1); for (let i = 0; i < 1000; i++) { const v = r.int(3, 5); expect(v >= 3 && v <= 5).toBe(true); }
+});
diff --git a/tests/core/time.test.ts b/tests/core/time.test.ts
new file mode 100644
index 0000000..c6481b7
--- /dev/null
+++ b/tests/core/time.test.ts
@@ -0,0 +1,8 @@
+import { test, expect } from "bun:test";
+import { ticksToSeconds, secondsToTicks } from "../../src/core/time.ts";
+
+test("ticks/seconds", () => {
+  expect(ticksToSeconds(960, 60)).toBeCloseTo(1);
+  expect(ticksToSeconds(3840, 120)).toBeCloseTo(2);
+  expect(secondsToTicks(0.5, 120)).toBeCloseTo(960);
+});