feat: release script builds Universal + ARM64 variants

Each release now produces two distribution zips:
- Scarf-vX.X.X-Universal.zip  (arm64 + x86_64, recommended)
- Scarf-vX.X.X-ARM64.zip      (arm64 only, ~14% smaller)

Both are independently archived, exported with Developer ID, notarized,
and stapled via a new build_variant helper. The appcast still points at
the Universal zip since it works on all supported macs; ARM64 is an
alternative manual download for Apple Silicon users who want the smaller
file.

README updated to list both variants.

Prompted by the v1.6.1 release shipping only Universal; the ARM64 zip
for v1.6.1 was produced ad-hoc and uploaded to the existing release.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -101,6 +101,7 @@ If a Hermes update changes the database schema or CLI output format, Scarf may n
 Download the latest build from [Releases](https://github.com/awizemann/scarf/releases):
 
 - `Scarf-vX.X.X-Universal.zip` — Apple Silicon + Intel (recommended)
+- `Scarf-vX.X.X-ARM64.zip` — Apple Silicon only (smaller download)
 
 1. Unzip and drag **Scarf.app** to Applications
 2. Launch normally — builds are Developer ID signed and notarized, so Gatekeeper accepts them on first launch