ci: drop macos-13 (deprecated) — cross-compile x86_64-apple-darwin from Apple Silicon

GitHub Actions deprecated macos-13 Intel runners. Apple Silicon (macos-latest)
can cargo build/check x86_64-apple-darwin out of the box (no cross tool needed).

- rust-multiplatform.yml: macos-x86_64 job uses macos-latest + check_only=true
  (binary arch ≠ host arch so no test runs; cargo check verifies the workspace
  type-checks for x86_64 Macs)
- rust-release.yml: macos-x86_64 job uses macos-latest, cargo build --release
  cross-compiles to x86_64; archive packaged as before

.github/workflows/rust-multiplatform.yml

@@ -36,10 +36,14 @@ jobs:
             runner: macos-latest
             target: aarch64-apple-darwin
             cross: false
+            # macos-13 (Intel runners) deprecated; build x86_64-apple-darwin via
+            # cross-compile from Apple Silicon. cargo build/check run; no test
+            # since binary arch ≠ host arch.
           - label: macos-x86_64
-            runner: macos-13
+            runner: macos-latest
             target: x86_64-apple-darwin
             cross: false
+            check_only: true
           - label: linux-x86_64
             runner: ubuntu-latest
             target: x86_64-unknown-linux-gnu
@@ -78,8 +82,11 @@ jobs:
         if: matrix.cross == true
         run: cargo install cross --locked --version 0.2.5
       - name: Build (host)
-        if: matrix.cross == false
+        if: matrix.cross == false && matrix.check_only != true
         run: cargo build --workspace --target ${{ matrix.target }} --release
+      - name: Check (cross-arch host, e.g. macos-x86_64 from Apple Silicon)
+        if: matrix.check_only == true
+        run: cargo check --workspace --target ${{ matrix.target }}
       - name: Build (cross)
         if: matrix.cross == true
         run: cross build --workspace --target ${{ matrix.target }} --release
@@ -99,10 +106,10 @@ jobs:
         # than the headless DRI fallback that fails `eglInitialize`.
         run: xvfb-run -a cargo test --workspace --target ${{ matrix.target }}
       - name: Test (host, macOS / Windows)
-        if: matrix.cross == false && runner.os != 'Linux'
+        if: matrix.cross == false && matrix.check_only != true && runner.os != 'Linux'
         run: cargo test --workspace --target ${{ matrix.target }}
       - name: Upload openpencil-app binary
-        if: matrix.cross == false
+        if: matrix.cross == false && matrix.check_only != true
         uses: actions/upload-artifact@v4
         with:
           name: openpencil-app-${{ matrix.label }}

.github/workflows/rust-release.yml

@@ -22,8 +22,11 @@ jobs:
             runner: macos-latest
             target: aarch64-apple-darwin
             archive: tar.gz
+          # macos-13 (Intel) deprecated; cross-compile x86_64-apple-darwin
+          # from Apple Silicon (cargo supports cross-compile to host's other
+          # arch out of the box, no `cross` needed).
           - label: macos-x86_64
-            runner: macos-13
+            runner: macos-latest
             target: x86_64-apple-darwin
             archive: tar.gz
           - label: linux-x86_64