feat(CompareBoard): add fitColumns honest column gating

src/features/CompareBoard/lib/measure/fitColumns.test.ts

@@ -0,0 +1,28 @@
+import {
+    describe,
+    expect,
+    it,
+} from 'vitest';
+import { fitColumns } from './fitColumns';
+
+describe('fitColumns', () => {
+    it('packs as many honest columns as fit, gap-aware', () => {
+        // each needs 600, gap 40, available 1280 -> 1 col=600, 2 cols=1240, 3=1880
+        expect(fitColumns({ naturalWidth: 600, available: 1280, gap: 40, maxColumns: 3 })).toBe(2);
+    });
+    it('never exceeds maxColumns even with room', () => {
+        expect(fitColumns({ naturalWidth: 100, available: 5000, gap: 20, maxColumns: 3 })).toBe(3);
+    });
+    it('never returns less than 1', () => {
+        expect(fitColumns({ naturalWidth: 9000, available: 300, gap: 20, maxColumns: 3 })).toBe(1);
+    });
+    it('fits a column at the exact boundary (inclusive)', () => {
+        // 2 cols: 2*600 + 1*40 = 1240 == available -> fits
+        expect(fitColumns({ naturalWidth: 600, available: 1240, gap: 40, maxColumns: 3 })).toBe(2);
+        // one px short -> only 1
+        expect(fitColumns({ naturalWidth: 600, available: 1239, gap: 40, maxColumns: 3 })).toBe(1);
+    });
+    it('respects a maxColumns of 1 even with unlimited room', () => {
+        expect(fitColumns({ naturalWidth: 100, available: 5000, gap: 20, maxColumns: 1 })).toBe(1);
+    });
+});