feat(CompareBoard): add measureRoleHeight via Pretext line count

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

@@ -0,0 +1,32 @@
+import {
+    describe,
+    expect,
+    it,
+    vi,
+} from 'vitest';
+import { measureRoleHeight } from './measureFrameHeight';
+
+describe('measureRoleHeight', () => {
+    it('multiplies pretext line count by sizePx*lineHeight', () => {
+        const layout = vi.fn().mockReturnValue({ lineCount: 3, height: 0 });
+        const prepared = {} as never;
+        // 3 lines * 20px * 1.5 = 90
+        expect(measureRoleHeight({ prepared, maxWidth: 600, sizePx: 20, lineHeight: 1.5 }, layout)).toBe(90);
+    });
+    it('passes width and pixel line-height into pretext layout', () => {
+        const layout = vi.fn().mockReturnValue({ lineCount: 1, height: 0 });
+        measureRoleHeight({ prepared: {} as never, maxWidth: 600, sizePx: 16, lineHeight: 1.25 }, layout);
+        expect(layout).toHaveBeenCalledWith(expect.anything(), 600, 16 * 1.25);
+    });
+    it('returns 0 when the text lays out to zero lines (empty specimen)', () => {
+        const layout = vi.fn().mockReturnValue({ lineCount: 0, height: 0 });
+        expect(measureRoleHeight({ prepared: {} as never, maxWidth: 600, sizePx: 16, lineHeight: 1.5 }, layout))
+            .toBe(0);
+    });
+    it('handles fractional sizes and line-heights without rounding', () => {
+        const layout = vi.fn().mockReturnValue({ lineCount: 2, height: 0 });
+        // 2 * 15.5 * 1.4 = 43.4
+        expect(measureRoleHeight({ prepared: {} as never, maxWidth: 320, sizePx: 15.5, lineHeight: 1.4 }, layout))
+            .toBeCloseTo(43.4);
+    });
+});

src/features/CompareBoard/lib/measure/measureFrameHeight.ts

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+import {
+    type PreparedText,
+    layout as pretextLayout,
+} from '@chenglou/pretext';
+
+/**
+ * Inputs for measuring one role block's rendered height.
+ */
+export interface RoleHeightInput {
+    /**
+     * Pretext-prepared specimen text for this role+font.
+     */
+    prepared: PreparedText;
+    /**
+     * Available width in px (the focal frame's content width).
+     */
+    maxWidth: number;
+    /**
+     * Resolved font-size in px.
+     */
+    sizePx: number;
+    /**
+     * Unitless line-height multiplier.
+     */
+    lineHeight: number;
+}
+
+/**
+ * Height in px of a role's text block at the given width, from Pretext's
+ * pure-arithmetic line count.
+ *
+ * Height is `lineCount * sizePx * lineHeight` rather than Pretext's own
+ * `height` so it tracks the CSS box model exactly (line-height as a multiple of
+ * font-size), keeping measurement and render in lockstep — the zero-shift
+ * invariant.
+ *
+ * @param input - Prepared text plus width and resolved type metrics.
+ * @param layout - Pretext layout fn; injectable for unit tests, defaults to
+ * `@chenglou/pretext`'s `layout`.
+ * @returns The block height in px.
+ */
+export function measureRoleHeight(input: RoleHeightInput, layout = pretextLayout): number {
+    const { prepared, maxWidth, sizePx, lineHeight } = input;
+    const { lineCount } = layout(prepared, maxWidth, sizePx * lineHeight);
+    return lineCount * sizePx * lineHeight;
+}