chore(release): drop Intel macOS build; arm64 only

The macos-13 / x86_64-apple-darwin matrix entry was failing in the
v0.3.5 run and we don't plan to ship Intel macOS binaries. Collapse
the macOS job to a single arm64 build on macos-14 (Apple Silicon),
removing the matrix and threading the target literally through the
build / .app / .dmg steps. README updates to match.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.github/workflows/release.yml

@@ -87,17 +87,7 @@ jobs:
           gh release upload ${{ github.ref_name }} H7CAD-${{ github.ref_name }}-windows-x86_64.exe --clobber --repo ${{ github.repository }}
 
   build-macos:
-    strategy:
-      fail-fast: false
-      matrix:
-        include:
-          - runner: macos-14    # Apple Silicon (arm64)
-            target: aarch64-apple-darwin
-            arch: arm64
-          - runner: macos-13    # Intel (x86_64)
-            target: x86_64-apple-darwin
-            arch: x86_64
-    runs-on: ${{ matrix.runner }}
+    runs-on: macos-14    # Apple Silicon (arm64) only
     permissions:
       contents: write
 
@@ -108,7 +98,7 @@ jobs:
       - name: Install Rust
         uses: dtolnay/rust-toolchain@stable
         with:
-          targets: ${{ matrix.target }}
+          targets: aarch64-apple-darwin
 
       - name: Install librsvg (for rsvg-convert)
         env:
@@ -116,7 +106,7 @@ jobs:
         run: brew install librsvg
 
       - name: Build
-        run: cargo build --release --target ${{ matrix.target }}
+        run: cargo build --release --target aarch64-apple-darwin
 
       - name: Build .icns from SVG
         run: |
@@ -137,7 +127,7 @@ jobs:
           APP=H7CAD.app
           rm -rf "$APP"
           mkdir -p "$APP/Contents/MacOS" "$APP/Contents/Resources"
-          cp target/${{ matrix.target }}/release/H7CAD "$APP/Contents/MacOS/H7CAD"
+          cp target/aarch64-apple-darwin/release/H7CAD "$APP/Contents/MacOS/H7CAD"
           chmod +x "$APP/Contents/MacOS/H7CAD"
           cp AppIcon.icns "$APP/Contents/Resources/AppIcon.icns"
           # Substitute version into Info.plist.
@@ -159,5 +149,5 @@ jobs:
         env:
           GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run: |
-          mv H7CAD.dmg H7CAD-${{ github.ref_name }}-macos-${{ matrix.arch }}.dmg
-          gh release upload ${{ github.ref_name }} H7CAD-${{ github.ref_name }}-macos-${{ matrix.arch }}.dmg --clobber --repo ${{ github.repository }}
+          mv H7CAD.dmg H7CAD-${{ github.ref_name }}-macos-arm64.dmg
+          gh release upload ${{ github.ref_name }} H7CAD-${{ github.ref_name }}-macos-arm64.dmg --clobber --repo ${{ github.repository }}