feat(FN-5608): add linux arm64 dual-arch support to desktop release workflo

Adds Linux ARM64 as a target architecture to the desktop release pipeline, including dual-arch release and test-release workflows, electron-builder configuration updates, and corresponding tests and documentation.

Fusion-Task-Id: FN-5608

Co-authored-by: Fusion (runfusion.ai) <noreply@runfusion.ai>
Fusion-Task-Id: FN-5608

packages/desktop/README.md

@@ -357,9 +357,10 @@ Desktop packaging is configured in `electron-builder.yml`.
   - Electron-updater feed files are also published per platform: `latest.yml` (Windows), `latest-mac.yml` (macOS), and `latest-linux.yml` (Linux). `setupAutoUpdater` / `triggerUpdateCheck` resolve these feeds from the GitHub Release channel.
   - Windows: x64 + arm64 outputs (NSIS + portable), matching `.exe.sha256` sidecars, and `.blockmap` files.
   - macOS: `Fusion-<version>-mac-arm64.dmg`, `Fusion-<version>-mac-x64.dmg`, matching `.zip` variants, `.sha256` sidecars, and `.blockmap` files.
-  - Linux: `Fusion-<version>-linux-x64.AppImage` with `.sha256`, plus best-effort `.deb` and `.tar.gz` outputs (and sidecars) when available on the runner image.
+  - Linux: `Fusion-<version>-linux-x64.AppImage` and `Fusion-<version>-linux-arm64.AppImage` with matching `.sha256` sidecars, plus best-effort `.deb` and `.tar.gz` outputs per arch (`Fusion-<version>-linux-x64.{deb,tar.gz}` / `Fusion-<version>-linux-arm64.{deb,tar.gz}`) and sidecars when available on the runner image.
 - Tag-less release rehearsal workflow (`.github/workflows/test-release.yml`) mirrors that artifact collection path without publishing a real GitHub Release.
-- Linux desktop artifacts now include detached GPG signature sidecars: `*.AppImage.asc`, `*.deb.asc`, and `*.tar.gz.asc` when Linux signing secrets are configured in CI.
+- Linux ARM64 artifacts are cross-built from the `ubuntu-latest` x64 runner by passing `electron-builder --linux --x64 --arm64`; running/validating arm64 installers still requires an arm64 Linux device or emulator.
+- Linux desktop artifacts can include detached GPG signature sidecars (`*.AppImage.asc`, `*.deb.asc`, `*.tar.gz.asc`) when Linux signing secrets are configured in CI; full Linux desktop code-signing rollout remains tracked in FN-5605.
 - Linux `.deb` and `.tar.gz` outputs are best-effort and may be absent on some runner images without failing the release.
 
 ### Linux signing

packages/desktop/electron-builder.yml

@@ -92,5 +92,14 @@ linux:
   category: Development
   target:
     - target: AppImage
+      arch:
+        - x64
+        - arm64
     - target: deb
+      arch:
+        - x64
+        - arm64
     - target: tar.gz
+      arch:
+        - x64
+        - arm64

packages/desktop/src/__tests__/electron-builder-config.test.ts

@@ -84,6 +84,27 @@ describe("electron-builder desktop config", () => {
       (match) => match[1]?.trim(),
     );
     expect(linuxTargetEntries[0]).toBe("AppImage");
+
+    const linuxArchMatches = Array.from(
+      linuxSection.matchAll(
+        /-\s*target:\s*(AppImage|deb|tar\.gz)\s*arch:\s*([\s\S]*?)(?=\n\s*-\s*target:|\n\w|$)/g,
+      ),
+    );
+
+    expect(linuxArchMatches).toHaveLength(3);
+
+    const linuxArchByTarget = new Map(
+      linuxArchMatches.map((match) => {
+        const target = match[1];
+        const archBlock = match[2] ?? "";
+        const archValues = Array.from(archBlock.matchAll(/-\s*(x64|arm64)/g), (entry) => entry[1]).sort();
+        return [target, archValues] as const;
+      }),
+    );
+
+    expect(linuxArchByTarget.get("AppImage")).toEqual(["arm64", "x64"]);
+    expect(linuxArchByTarget.get("deb")).toEqual(["arm64", "x64"]);
+    expect(linuxArchByTarget.get("tar.gz")).toEqual(["arm64", "x64"]);
   });
 });
 

packages/desktop/src/__tests__/release-workflow.test.ts

@@ -32,6 +32,10 @@ describe("desktop release workflow wiring", () => {
       expect(workflow).toContain("build-desktop-linux:");
       expect(workflow).toContain("runs-on: ubuntu-latest");
       expect(workflow).toMatch(/pnpm --filter @fusion\/desktop dist:linux|electron-builder --linux/);
+      expect(workflow).toContain("--x64");
+      expect(workflow).toContain("--arm64");
+      expect(workflow).toContain("Fusion-*-linux-arm64.AppImage");
+      expect(workflow).toContain("Fusion-*-linux-x64.AppImage");
       expect(workflow).toContain("name: fusion-desktop-linux");
       expect(workflow).toContain("packages/desktop/dist-electron/latest-linux.yml");
     }