fix(embedded-mpv): hash runtime cache identities

apps/electron-backend/src/app/services/embedded-mpv-frame-copy-runtime.spec.ts

@@ -640,6 +640,35 @@ describe('embedded-mpv-frame-copy-runtime', () => {
         expect(fileSystem?.readFileSync).toHaveBeenCalled();
     });
 
+    it('invalidates a cached result when helper or manifest bytes change under identical stats', () => {
+        const fixture = createFixture(rootDir);
+        const fixedStats = new Map([
+            [fixture.helperPath, lstatSync(fixture.helperPath)],
+            [fixture.manifestPath, lstatSync(fixture.manifestPath)],
+        ]);
+        fileSystem = {
+            ...fileSystem,
+            lstatSync: jest.fn(
+                (filePath: string) =>
+                    fixedStats.get(filePath) ?? lstatSync(filePath)
+            ),
+        };
+        const probeRuntime = createProbe();
+
+        expect(probeRuntime(fixture.helperPath).usable).toBe(true);
+
+        const changedHelper = readFileSync(fixture.helperPath);
+        changedHelper[0] ^= 0xff;
+        writeFileSync(fixture.helperPath, changedHelper);
+        expect(probeRuntime(fixture.helperPath).usable).toBe(true);
+
+        fixture.manifest.generatedAt = '2026-07-18T00:00:00.000Z';
+        writeManifest(fixture.manifestPath, fixture.manifest);
+        expect(probeRuntime(fixture.helperPath).usable).toBe(true);
+
+        expect(spawnRuntimeProbe).toHaveBeenCalledTimes(3);
+    });
+
     it.each(['portable', 'flatpak'] as const)(
         'validates the exact %s bundled closure and uses only its private library directory',
         (profile) => {

apps/electron-backend/src/app/services/embedded-mpv-frame-copy-runtime.ts

@@ -438,7 +438,11 @@ function isMissingFileError(error: unknown): boolean {
     );
 }
 
-function fileIdentity(filePath: string, stat: nodeFileSystem.Stats): string {
+function fileIdentity(
+    filePath: string,
+    stat: nodeFileSystem.Stats,
+    contents: Buffer
+): string {
     return [
         path.resolve(filePath),
         stat.dev,
@@ -447,6 +451,7 @@ function fileIdentity(filePath: string, stat: nodeFileSystem.Stats): string {
         stat.size,
         stat.mtimeMs,
         stat.ctimeMs,
+        createHash('sha256').update(contents).digest('hex'),
     ].join(':');
 }
 
@@ -1546,10 +1551,25 @@ export function createEmbeddedMpvFrameCopyRuntimeProbe(
             );
         }
 
+        let helperContents: Buffer;
+        let manifestContents: Buffer;
+        try {
+            helperContents = dependencies.fileSystem.readFileSync(helperPath);
+        } catch {
+            return failure('runtime-artifact-invalid');
+        }
+        try {
+            manifestContents =
+                dependencies.fileSystem.readFileSync(manifestPath);
+        } catch {
+            return failure('runtime-manifest-invalid');
+        }
+
         const cacheKey = `${manifestContract}\0${fileIdentity(
             helperPath,
-            helperStat
-        )}\0${fileIdentity(manifestPath, manifestStat)}`;
+            helperStat,
+            helperContents
+        )}\0${fileIdentity(manifestPath, manifestStat, manifestContents)}`;
         const cached = resultCache.get(cacheKey);
         if (cached) {
             return cached;