Sign the macOS bundle, or it arrives "damaged"

The first release could not be opened: macOS said "WarpEngine Store is damaged
and can't be opened. You should move it to the Bin."

Not a wording problem — an integrity one. electron-builder found no signing
identity and skipped signing, so the bundle kept only the linker's ad-hoc
signature on its main executable, with no resource seal. `codesign --verify` said
"code has no resources but signature indicates they must be present", and
Gatekeeper reports that as damaged and offers no way past it, unlike an
un-notarised app which can at least be approved.

`scripts/after-pack.js` now signs the bundle itself during packaging. Measured on
a copy unzipped from the artifact with the quarantine flag set by hand:

  before  code has no resources but signature indicates they must be present
  after   valid on disk; satisfies its Designated Requirement

and the identifier is ours rather than `Electron`. `syspolicy_check` is down to
its expected "adhoc signed" warning. A downloaded copy still has to be approved —
that is Gatekeeper policy for anything un-notarised, and notarisation needs a paid
Developer ID — so the README and the release notes lead with the one command that
does it.

Two smaller things the failure turned up:

- The self-test was passing silently. With a copy of the app already open, the
  second process lost the single-instance lock and exited 0 with no output, which
  reads exactly like success. It now uses its own user-data directory and skips
  the lock, and it caught a real launch failure immediately afterwards.
- The README claimed right-click ▸ Open was enough. It was not, and I had not
  checked it — replaced with what the measurements support.

v1.0.0's attachments are withdrawn rather than left downloadable.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

package.json

@@ -1,7 +1,7 @@
 {
   "name": "warp-engine-desktop-gui",
   "productName": "WarpEngine Store",
-  "version": "1.0.0",
+  "version": "1.0.1",
   "description": "Graphical client for a WarpEngine desktop store: install the catalog into your own application menu.",
   "license": "MIT",
   "author": "Teletype Games <games@teletype.hu>",
@@ -51,7 +51,8 @@
         "AppImage",
         "deb"
       ]
-    }
+    },
+    "afterPack": "scripts/after-pack.js"
   },
   "allowScripts": {
     "electron@43.4.0": true