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.

.github/workflows/apple-release.yml

@@ -0,0 +1,230 @@
+name: Apple release (macOS DMG)
+
+# Builds, signs, notarizes, and publishes the direct-download macOS build:
+#   - a Developer ID–signed, notarized VniDrop-<version>.dmg,
+#   - a Sparkle appcast.xml (both attached to the GitHub Release), and
+#   - an updated Homebrew cask pushed to the sudosylabs/homebrew-vnidrop tap.
+#
+# The App Store / TestFlight build is NOT produced here — that goes through Xcode
+# Organizer / App Store Connect. This workflow only covers direct distribution.
+#
+# Trigger: push a tag vMAJOR.MINOR.PATCH (must point at a commit on master), or
+# run manually with an explicit version (produces artifacts, no Release).
+
+on:
+  push:
+    tags:
+      - "v*.*.*"
+  workflow_dispatch:
+    inputs:
+      version:
+        description: Release version in MAJOR.MINOR.PATCH form
+        required: true
+        default: "0.1.0"
+        type: string
+
+permissions:
+  contents: read
+
+concurrency:
+  group: apple-release-${{ github.ref }}
+  cancel-in-progress: false
+
+defaults:
+  run:
+    shell: bash
+
+jobs:
+  build:
+    name: Build & notarize DMG
+    runs-on: macos-latest
+    timeout-minutes: 90
+    permissions:
+      contents: write
+    outputs:
+      version: ${{ steps.version.outputs.app }}
+    steps:
+      - name: Checkout
+        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+        with:
+          fetch-depth: 0
+          persist-credentials: false
+
+      - name: Verify tag is on master
+        if: github.event_name == 'push'
+        run: |
+          if ! git merge-base --is-ancestor "$GITHUB_SHA" origin/master; then
+            echo "Release tags must point to a commit on master" >&2
+            exit 1
+          fi
+
+      - name: Resolve version
+        id: version
+        env:
+          REQUESTED_VERSION: ${{ inputs.version || '' }}
+        run: |
+          if [ "${GITHUB_REF_TYPE:-}" = "tag" ]; then
+            version="${GITHUB_REF_NAME#v}"
+          else
+            version="$REQUESTED_VERSION"
+          fi
+          [[ "$version" =~ ^[0-9]+\.[0-9]+\.[0-9]+$ ]] || { echo "bad version '$version'" >&2; exit 1; }
+          echo "app=$version" >> "$GITHUB_OUTPUT"
+
+      - name: Select Xcode
+        run: sudo xcode-select -s /Applications/Xcode.app
+
+      - name: Install Rust toolchain
+        uses: dtolnay/rust-toolchain@29eef336d9b2848a0b548edc03f92a220660cdb8 # v1
+        with:
+          toolchain: stable
+          targets: aarch64-apple-darwin
+
+      - name: Cache Cargo
+        uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            target
+          key: apple-release-cargo-${{ hashFiles('Cargo.lock') }}
+          restore-keys: apple-release-cargo-
+
+      - name: Install tooling
+        run: brew install xcodegen swiftlint create-dmg
+
+      - name: Install Bun
+        uses: oven-sh/setup-bun@v2
+
+      - name: Download Sparkle tools
+        # generate_appcast + sign_update ship in the Sparkle release tarball.
+        run: |
+          set -euo pipefail
+          ver="2.9.4"
+          curl -fsSL -o /tmp/sparkle.tar.xz \
+            "https://github.com/sparkle-project/Sparkle/releases/download/${ver}/Sparkle-${ver}.tar.xz"
+          mkdir -p /tmp/sparkle && tar -xJf /tmp/sparkle.tar.xz -C /tmp/sparkle
+          echo "SPARKLE_BIN=/tmp/sparkle/bin" >> "$GITHUB_ENV"
+
+      - name: Import Developer ID certificate
+        env:
+          CERT_P12_BASE64: ${{ secrets.DEVELOPER_ID_CERT_P12 }}
+          CERT_PASSWORD: ${{ secrets.DEVELOPER_ID_CERT_PASSWORD }}
+        run: |
+          set -euo pipefail
+          keychain="$RUNNER_TEMP/signing.keychain-db"
+          kpw="$(openssl rand -hex 20)"
+          security create-keychain -p "$kpw" "$keychain"
+          security set-keychain-settings -lut 21600 "$keychain"
+          security unlock-keychain -p "$kpw" "$keychain"
+          echo "$CERT_P12_BASE64" | base64 --decode > "$RUNNER_TEMP/cert.p12"
+          security import "$RUNNER_TEMP/cert.p12" -k "$keychain" -P "$CERT_PASSWORD" \
+            -T /usr/bin/codesign -T /usr/bin/security
+          security set-key-partition-list -S apple-tool:,apple:,codesign: -s -k "$kpw" "$keychain"
+          # Prepend our keychain so codesign/xcodebuild can find the identity.
+          security list-keychains -d user -s "$keychain" $(security list-keychains -d user | tr -d '"')
+          rm -f "$RUNNER_TEMP/cert.p12"
+
+      - name: Store notarytool credentials
+        env:
+          NOTARY_KEY_P8: ${{ secrets.NOTARY_API_KEY }}
+          NOTARY_KEY_ID: ${{ secrets.NOTARY_KEY_ID }}
+          NOTARY_ISSUER: ${{ secrets.NOTARY_ISSUER }}
+        run: |
+          set -euo pipefail
+          echo "$NOTARY_KEY_P8" | base64 --decode > "$RUNNER_TEMP/notary.p8"
+          xcrun notarytool store-credentials vnidrop-notary \
+            --key "$RUNNER_TEMP/notary.p8" \
+            --key-id "$NOTARY_KEY_ID" \
+            --issuer "$NOTARY_ISSUER"
+          echo "NOTARY_PROFILE=vnidrop-notary" >> "$GITHUB_ENV"
+
+      - name: Write Sparkle signing key
+        env:
+          SPARKLE_ED_PRIVATE_KEY: ${{ secrets.SPARKLE_ED_PRIVATE_KEY }}
+        run: |
+          printf '%s' "$SPARKLE_ED_PRIVATE_KEY" > "$RUNNER_TEMP/sparkle_ed_private_key"
+          echo "SPARKLE_ED_KEY_FILE=$RUNNER_TEMP/sparkle_ed_private_key" >> "$GITHUB_ENV"
+
+      - name: Build, sign & notarize DMG
+        run: apple/scripts/build-dmg.sh "${{ steps.version.outputs.app }}"
+
+      - name: Generate appcast
+        env:
+          RELEASE_REPO: ${{ github.repository }}
+        run: apple/scripts/generate-appcast.sh "${{ steps.version.outputs.app }}"
+
+      - name: Upload artifacts
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
+        with:
+          name: vnidrop-${{ steps.version.outputs.app }}-macos-dmg
+          path: |
+            apple/dist/VniDrop-*.dmg
+            apple/dist/appcast.xml
+          if-no-files-found: error
+          retention-days: 14
+
+      - name: Publish GitHub Release
+        if: github.event_name == 'push' && github.ref_type == 'tag'
+        env:
+          GH_TOKEN: ${{ github.token }}
+          GH_REPO: ${{ github.repository }}
+        run: |
+          set -euo pipefail
+          tag="$GITHUB_REF_NAME"
+          version="${tag#v}"
+          if gh release view "$tag" >/dev/null 2>&1; then
+            echo "Release $tag already exists; refusing to replace assets" >&2
+            exit 1
+          fi
+          gh release create "$tag" \
+            "apple/dist/VniDrop-${version}.dmg" \
+            "apple/dist/appcast.xml" \
+            --verify-tag \
+            --title "VniDrop $version" \
+            --generate-notes
+
+  update-cask:
+    name: Update Homebrew cask
+    needs: build
+    if: github.event_name == 'push' && github.ref_type == 'tag'
+    runs-on: ubuntu-22.04
+    timeout-minutes: 15
+    steps:
+      - name: Checkout
+        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+        with:
+          persist-credentials: false
+
+      - name: Download DMG artifact
+        uses: actions/download-artifact@70fc10c6e5e1ce46ad2ea6f2b72d43f7d47b13c3 # v8.0.0
+        with:
+          name: vnidrop-${{ needs.build.outputs.version }}-macos-dmg
+          path: dist
+
+      - name: Render cask
+        env:
+          VERSION: ${{ needs.build.outputs.version }}
+        run: |
+          set -euo pipefail
+          sha="$(sha256sum "dist/VniDrop-${VERSION}.dmg" | cut -d' ' -f1)"
+          sed -e "s/^  version \".*\"/  version \"${VERSION}\"/" \
+              -e "s/^  sha256 \".*\"/  sha256 \"${sha}\"/" \
+              packaging/homebrew/vnidrop.rb > /tmp/vnidrop.rb
+          echo "Rendered cask:"; cat /tmp/vnidrop.rb
+
+      - name: Push to tap
+        env:
+          TAP_TOKEN: ${{ secrets.HOMEBREW_TAP_TOKEN }}
+          VERSION: ${{ needs.build.outputs.version }}
+        run: |
+          set -euo pipefail
+          git clone "https://x-access-token:${TAP_TOKEN}@github.com/sudosylabs/homebrew-vnidrop.git" tap
+          mkdir -p tap/Casks
+          cp /tmp/vnidrop.rb tap/Casks/vnidrop.rb
+          cd tap
+          git config user.name "vnidrop-release-bot"
+          git config user.email "release-bot@users.noreply.github.com"
+          git add Casks/vnidrop.rb
+          git commit -m "vnidrop ${VERSION}" || { echo "no cask changes"; exit 0; }
+          git push