feat: core types, time, rng, model fetch

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

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;
+}