Add scaled number formatter for K/M/B values

lib/format.test.ts

@@ -0,0 +1,29 @@
+import { describe, expect, it } from 'bun:test';
+import { formatScaledNumber } from './format';
+
+describe('formatScaledNumber', () => {
+  it('keeps values below one thousand unscaled', () => {
+    expect(formatScaledNumber(950)).toBe('950');
+    expect(formatScaledNumber(950.44, { maximumFractionDigits: 2 })).toBe('950.44');
+  });
+
+  it('scales thousands with K suffix', () => {
+    expect(formatScaledNumber(12_345)).toBe('12.3K');
+  });
+
+  it('scales millions with M suffix', () => {
+    expect(formatScaledNumber(3_250_000)).toBe('3.3M');
+  });
+
+  it('scales billions with B suffix', () => {
+    expect(formatScaledNumber(7_500_000_000)).toBe('7.5B');
+  });
+
+  it('preserves negative sign when scaled', () => {
+    expect(formatScaledNumber(-2_750_000)).toBe('-2.8M');
+  });
+
+  it('promotes rounded values to the next scale when needed', () => {
+    expect(formatScaledNumber(999_950)).toBe('1M');
+  });
+});