ci(release): run release-pr / release jobs on macos-26

The release jobs run `cargo publish --verify`, which fresh-builds the
crate against crates.io versions of every dependency. The current
crates.io `apple-metal` (v0.6.1) statically references macOS 26 SDK
Metal symbols (MTLSamplerDescriptor.reductionMode, etc.), so the
verify step needs the macOS 26 SDK to compile. `macos-latest` is
pinned to macOS 14 in GitHub Actions today and ships an older Xcode
SDK, so the v3.1.2 publish failed with:

    cannot find 'MTLSamplerReductionMode' in scope
    value of type 'MTLSamplerDescriptor' has no member 'reductionMode'
    error: failed to verify package tarball

Move release-pr + release to macos-26 — same SDK pin we already use
for the build matrix and lint job. This lets release-plz actually
publish v3.1.2 to crates.io on the next push to main.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 1313

.github/workflows/ci.yml

@@ -156,10 +156,12 @@ jobs:
         run: cargo run --example 15_memory_leak_check --features "${{ matrix.features }}"
 
   # ==========================================================================
-  # Release (only on main, after successful build and leak-check)
+  # Release (only on main, after successful build and leak-check).
+  # Pinned to macos-26 so `cargo publish --verify` builds against the macOS
+  # 26 SDK that apple-metal 0.6.x already requires (see build matrix note).
   # ==========================================================================
   release-pr:
-    runs-on: macos-latest
+    runs-on: macos-26
     needs: [lint, build, leak-check]
     if: github.ref == 'refs/heads/main' && github.event_name == 'push'
     environment: release
@@ -185,7 +187,7 @@ jobs:
           CARGO_REGISTRY_TOKEN: ${{ secrets.CARGO_REGISTRY_TOKEN }}
 
   release:
-    runs-on: macos-latest
+    runs-on: macos-26
     needs: [lint, build, leak-check]
     if: github.ref == 'refs/heads/main' && github.event_name == 'push'
     environment: release