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/scripts/generate-appcast.sh

@@ -0,0 +1,68 @@
+#!/usr/bin/env bash
+#
+# Generates/updates the Sparkle appcast for the direct-download build. Runs
+# Sparkle's `generate_appcast` over the DMGs in apple/dist/, writing:
+#   - apple/dist/appcast.xml
+#
+# The <enclosure> URLs point at the matching GitHub Release download assets, and
+# each item is signed with the project's EdDSA key (from a key file or the
+# keychain). The resulting appcast.xml is uploaded as a release asset; the app's
+# SUFeedURL (/releases/latest/download/appcast.xml) always resolves to the newest.
+#
+# Usage: apple/scripts/generate-appcast.sh [version]
+#
+# Environment:
+#   DIST_DIR              Folder holding the DMG(s). Default: apple/dist
+#   SPARKLE_BIN           Dir containing generate_appcast. Auto-located when unset.
+#   SPARKLE_ED_KEY_FILE   Path to the EdDSA private key file. When unset,
+#                         generate_appcast reads the key from the login keychain.
+#   RELEASE_REPO          owner/repo for enclosure URLs. Default: sudosylabs/vnidrop
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+REPO_ROOT="$(cd "$SCRIPT_DIR/../.." && pwd)"
+APPLE_DIR="$REPO_ROOT/apple"
+DIST_DIR="${DIST_DIR:-$APPLE_DIR/dist}"
+RELEASE_REPO="${RELEASE_REPO:-sudosylabs/vnidrop}"
+
+VERSION="${1:-}"
+if [ -z "$VERSION" ] && [ "${GITHUB_REF_TYPE:-}" = "tag" ]; then
+	VERSION="${GITHUB_REF_NAME#v}"
+fi
+[ -n "$VERSION" ] || { echo "error: version required (arg or tag)" >&2; exit 1; }
+
+# Enclosure URLs resolve to the specific release's assets.
+DOWNLOAD_PREFIX="https://github.com/$RELEASE_REPO/releases/download/v$VERSION"
+
+# --- Locate generate_appcast -------------------------------------------------
+find_tool() {
+	local name="$1"
+	if [ -n "${SPARKLE_BIN:-}" ] && [ -x "$SPARKLE_BIN/$name" ]; then
+		printf '%s' "$SPARKLE_BIN/$name"; return 0
+	fi
+	if command -v "$name" >/dev/null 2>&1; then command -v "$name"; return 0; fi
+	# Sparkle SPM artifact bundle lands under DerivedData SourcePackages.
+	local dd="${APPLE_DERIVED_DATA:-$HOME/Library/Developer/Xcode/DerivedData}"
+	local hit
+	hit="$(find "$dd" "$HOME/Library/Caches/org.swift.swiftpm" -type f -name "$name" \
+		-perm -111 2>/dev/null | head -1 || true)"
+	[ -n "$hit" ] && { printf '%s' "$hit"; return 0; }
+	return 1
+}
+GENERATE_APPCAST="$(find_tool generate_appcast || true)"
+if [ -z "$GENERATE_APPCAST" ]; then
+	echo "error: generate_appcast not found. Set SPARKLE_BIN to Sparkle's bin/ dir" >&2
+	echo "       (download from https://github.com/sparkle-project/Sparkle/releases)." >&2
+	exit 1
+fi
+echo "==> Using $GENERATE_APPCAST"
+
+# --- Generate ----------------------------------------------------------------
+args=( --download-url-prefix "$DOWNLOAD_PREFIX/" -o "$DIST_DIR/appcast.xml" )
+if [ -n "${SPARKLE_ED_KEY_FILE:-}" ]; then
+	args+=( --ed-key-file "$SPARKLE_ED_KEY_FILE" )
+fi
+echo "==> Generating appcast (v$VERSION) → $DIST_DIR/appcast.xml"
+"$GENERATE_APPCAST" "${args[@]}" "$DIST_DIR"
+
+echo "==> Done. Enclosure prefix: $DOWNLOAD_PREFIX/"