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.

apple/VniDrop/Resources/VniDropDirect.entitlements

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<!-- Entitlements for the direct-download (Developer ID + notarized) macOS build.
+
+     Deliberately NOT sandboxed: a sandboxed app signed for Developer ID requires a
+     provisioning profile, whereas direct-distribution apps run outside the App
+     Store sandbox by convention. Gatekeeper trust here comes from the hardened
+     runtime (ENABLE_HARDENED_RUNTIME) plus notarization, not the sandbox. The App
+     Store target (VniDrop) keeps VniDrop.entitlements with the sandbox enabled.
+
+     Networking and user file access need no entitlements once unsandboxed. -->
+<dict>
+</dict>
+</plist>