feat: core types, time, rng, model fetch

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

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);
+});