refactor(api): move RandomSource to shared and add rollInclusive

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

apps/api/src/shared/roll-range.spec.ts

@@ -0,0 +1,28 @@
+import type { RandomSource } from './random-source';
+import { rollInclusive } from './roll-range';
+
+function fixed(...values: number[]): RandomSource {
+  let index = 0;
+  return { next: () => values[index++] };
+}
+
+describe('rollInclusive', () => {
+  it('maps the bottom of the random range to min and the top to max', () => {
+    expect(rollInclusive(fixed(0), 4, 7)).toBe(4);
+    expect(rollInclusive(fixed(0.999), 4, 7)).toBe(7);
+  });
+
+  it('spreads the random range evenly across every value in between', () => {
+    expect(rollInclusive(fixed(0.25), 4, 7)).toBe(5);
+    expect(rollInclusive(fixed(0.5), 4, 7)).toBe(6);
+    expect(rollInclusive(fixed(0.5), 9, 15)).toBe(12);
+  });
+
+  it('never exceeds max even if the source yields exactly 1', () => {
+    expect(rollInclusive(fixed(1), 9, 15)).toBe(15);
+  });
+
+  it('returns the single value when min equals max', () => {
+    expect(rollInclusive(fixed(0.7), 1, 1)).toBe(1);
+  });
+});