ci: bump actions/checkout v4 -> v6 (Node 24 readiness)

GitHub forces the Node 24 runtime on 2026-06-16; checkout@v4 runs on the deprecated Node 20. checkout v6.0.2 declares runs.using: node24. All 9 usages (5 in build.yml, 4 in release.yml) are bare checkouts with no inputs, so the major bump is a drop-in.

Still on Node-20-era majors in release.yml, deferred (multi-major jumps with breaking changes, and release.yml only runs on tag push): upload-artifact v4->v7, download-artifact v4->v8, softprops/action-gh-release v2->v3.

.github/workflows/release.yml

@@ -32,7 +32,7 @@ jobs:
     name: build (${{ matrix.target }})
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: install build deps
         run: |
@@ -71,7 +71,7 @@ jobs:
     container:
       image: alpine:latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: install build deps
         run: apk add --no-cache build-base linux-headers tar
       - name: build static (musl)
@@ -111,7 +111,7 @@ jobs:
     runs-on: ubuntu-latest
     name: build (arm64-static / musl)
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: run dockcross arm64-musl build
         run: |
           # Fetch the dockcross wrapper script (handles UID/GID,
@@ -141,7 +141,7 @@ jobs:
     needs: [build, build-static-x86_64, build-static-arm64]
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - uses: actions/download-artifact@v4
         with: