chore(ci): bump actions/checkout and artifact actions to v5

GitHub flagged checkout@v4 / upload-artifact@v4 / download-artifact@v4
as Node.js 20 actions, force-migrated to Node 24 on 2026-06-02. Bump
all nine references to v5 ahead of the deadline. The artifact steps are
v5-compatible: upload uses a unique matrix-target name and the download
step flattens subdirectories with find afterward.

.github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
     name: Test
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
       - uses: dtolnay/rust-toolchain@stable
       - uses: Swatinem/rust-cache@v2
       - run: cargo test --workspace
@@ -23,7 +23,7 @@ jobs:
     name: Lint
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
       - uses: dtolnay/rust-toolchain@stable
         with:
           components: clippy, rustfmt
@@ -35,7 +35,7 @@ jobs:
     name: WASM
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
       - uses: dtolnay/rust-toolchain@stable
         with:
           targets: wasm32-unknown-unknown
@@ -50,7 +50,7 @@ jobs:
     name: Docs
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
       - uses: dtolnay/rust-toolchain@stable
       - uses: Swatinem/rust-cache@v2
       - run: cargo doc --no-deps --workspace