feat(apple): add direct-download macOS channel (notarized DMG + Sparkle + Homebrew)

Add a second macOS shipping channel alongside the App Store build:

- New VniDropDirect target (Release-Direct config) sharing VniDrop's sources via
  an AppBase target template; links Sparkle behind the DIRECT_DISTRIBUTION flag so
  the App Store binary never bundles a self-updater. arm64-only (core is arm64).
- Sparkle updater (SparkleUpdater.swift) + "Check for Updates" menu, compiled only
  under DIRECT_DISTRIBUTION; Info.plist SUFeedURL points at the GitHub Release
  /latest/download/appcast.xml, non-sandboxed entitlements for Developer ID.
- build-dmg.sh (archive → Developer ID export → DMG → sign → notarize → staple),
  generate-appcast.sh, and ExportOptions-DeveloperID.plist.
- apple-release.yml: on tag v*.*.*, build/notarize the DMG, publish the GitHub
  Release with appcast, and push the Homebrew cask to sudosylabs/homebrew-vnidrop.
  apple.yml gains a PR compile-check of the direct target.
- CFBundleVersion is stamped at build time as a UTC YYMMDD.HHMM timestamp for both
  channels, replacing the hand-maintained build number.
- Docs (RELEASE-MACOS.md, README), cask template + tap README, localized
  updates_check string, Makefile targets, gitignore for dist/ artifacts.

packaging/homebrew/vnidrop.rb

@@ -0,0 +1,36 @@
+# Homebrew cask for the direct-download (notarized .dmg) macOS build.
+#
+# This file is the source template. The Apple release workflow substitutes the
+# version + sha256 for each release and pushes the result to the tap repo
+# (sudosylabs/homebrew-vnidrop, path Casks/vnidrop.rb). Users then install with:
+#   brew install --cask sudosylabs/vnidrop/vnidrop
+#
+# `auto_updates true` tells Homebrew that the app updates itself via Sparkle, so
+# `brew upgrade` won't fight the in-app updater.
+cask "vnidrop" do
+  version "0.0.0"
+  sha256 "0000000000000000000000000000000000000000000000000000000000000000"
+
+  url "https://github.com/sudosylabs/vnidrop/releases/download/v#{version}/VniDrop-#{version}.dmg",
+      verified: "github.com/sudosylabs/vnidrop/"
+  name "VniDrop"
+  desc "Direct device-to-device file and folder transfer over the network"
+  homepage "https://github.com/sudosylabs/vnidrop"
+
+  livecheck do
+    url :url
+    strategy :github_latest
+  end
+
+  auto_updates true
+  depends_on arch: :arm64
+  depends_on macos: ">= :sequoia"
+
+  app "VniDrop.app"
+
+  zap trash: [
+    "~/Library/Application Support/com.vnidrop.app",
+    "~/Library/Caches/com.vnidrop.app",
+    "~/Library/Preferences/com.vnidrop.app.plist",
+  ]
+end