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/tap-README.md

@@ -0,0 +1,54 @@
+# homebrew-vnidrop
+
+Homebrew tap for [VniDrop](https://github.com/sudosylabs/vnidrop) — direct,
+private device-to-device file and folder transfer for macOS.
+
+> This repo only holds the Homebrew **cask**. The app itself lives at
+> [sudosylabs/vnidrop](https://github.com/sudosylabs/vnidrop). The cask here is
+> updated automatically by VniDrop's release pipeline on each tagged release.
+
+## Install
+
+```sh
+brew tap sudosylabs/vnidrop
+brew install --cask vnidrop
+```
+
+Or in one line:
+
+```sh
+brew install --cask sudosylabs/vnidrop/vnidrop
+```
+
+## Update
+
+VniDrop updates itself in-app via [Sparkle](https://sparkle-project.org), so you
+normally don't need to do anything. To update through Homebrew instead:
+
+```sh
+brew upgrade --cask vnidrop
+```
+
+## Uninstall
+
+```sh
+brew uninstall --cask vnidrop
+```
+
+Add `--zap` to also remove VniDrop's application support, cache, and preference
+files:
+
+```sh
+brew uninstall --zap --cask vnidrop
+```
+
+## Requirements
+
+- macOS 15 (Sequoia) or later, Apple Silicon.
+
+## What you get
+
+The cask installs the Developer ID–signed, notarized `VniDrop.app` from the
+matching [GitHub Release](https://github.com/sudosylabs/vnidrop/releases). App
+Store users should install from the Mac App Store instead — that build does not
+include the Sparkle self-updater.