feat: core types, time, rng, model fetch

Co-Authored-By: Claude Haiku 5.5 <[email protected]>

src/core/rng.ts

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+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;
+    },
+  };
+}

src/core/time.ts

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+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;
+}

src/core/types.ts

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+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];