refactor: extract mpv render metrics runtime apply helper

src/core/services/mpv-render-metrics-service.test.ts

@@ -0,0 +1,37 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { MpvSubtitleRenderMetrics } from "../../types";
+import { applyMpvSubtitleRenderMetricsPatchService } from "./mpv-render-metrics-service";
+
+const BASE: MpvSubtitleRenderMetrics = {
+  subPos: 100,
+  subFontSize: 38,
+  subScale: 1,
+  subMarginY: 34,
+  subMarginX: 19,
+  subFont: "sans-serif",
+  subSpacing: 0,
+  subBold: false,
+  subItalic: false,
+  subBorderSize: 2.5,
+  subShadowOffset: 0,
+  subAssOverride: "yes",
+  subScaleByWindow: true,
+  subUseMargins: true,
+  osdHeight: 720,
+  osdDimensions: null,
+};
+
+test("applyMpvSubtitleRenderMetricsPatchService returns unchanged on empty patch", () => {
+  const { next, changed } = applyMpvSubtitleRenderMetricsPatchService(BASE, {});
+  assert.equal(changed, false);
+  assert.deepEqual(next, BASE);
+});
+
+test("applyMpvSubtitleRenderMetricsPatchService reports changed when patch modifies value", () => {
+  const { next, changed } = applyMpvSubtitleRenderMetricsPatchService(BASE, {
+    subPos: 95,
+  });
+  assert.equal(changed, true);
+  assert.equal(next.subPos, 95);
+});