feat(font): compute split index and 3-region slice

src/entities/Font/lib/dualFontLayout/computeLineRenderModel/computeLineRenderModel.test.ts

@@ -42,4 +42,46 @@ describe('computeLineRenderModel', () => {
         expect(model.windowChars).toEqual([]);
         expect(model.rightText).toBe('');
     });
+
+    it('places entire line in rightText when slider is at 0', () => {
+        const line = makeLine([
+            { char: 'A', widthA: 10, widthB: 10 },
+            { char: 'B', widthA: 10, widthB: 10 },
+            { char: 'C', widthA: 10, widthB: 10 },
+        ]);
+        const model = computeLineRenderModel(line, 0, 500, 0);
+        expect(model.leftText).toBe('');
+        expect(model.windowChars).toEqual([]);
+        expect(model.rightText).toBe('ABC');
+    });
+
+    it('places entire line in leftText when slider is at 100', () => {
+        const line = makeLine([
+            { char: 'A', widthA: 10, widthB: 10 },
+            { char: 'B', widthA: 10, widthB: 10 },
+            { char: 'C', widthA: 10, widthB: 10 },
+        ]);
+        const model = computeLineRenderModel(line, 100, 500, 0);
+        expect(model.leftText).toBe('ABC');
+        expect(model.windowChars).toEqual([]);
+        expect(model.rightText).toBe('');
+    });
+
+    it('splits line correctly with slider mid-line (window=0)', () => {
+        // Equal widths → line is centered. Container=300, total=30 → xOffset=135.
+        // Char thresholds (per the threshold formula in the design):
+        //   threshold[i] = xOffset + prefA[i] + widthA[i]/2
+        //   i=0: 135 + 0  + 5  = 140  → 140/300 = 46.67%
+        //   i=1: 135 + 10 + 5  = 150  → 150/300 = 50.00%
+        //   i=2: 135 + 20 + 5  = 160  → 160/300 = 53.33%
+        const line = makeLine([
+            { char: 'A', widthA: 10, widthB: 10 },
+            { char: 'B', widthA: 10, widthB: 10 },
+            { char: 'C', widthA: 10, widthB: 10 },
+        ]);
+        // Slider just past B's threshold (50%) but not C's (53.33%).
+        const model = computeLineRenderModel(line, 51, 300, 0);
+        expect(model.leftText).toBe('AB');
+        expect(model.rightText).toBe('C');
+    });
 });