ci: standardize actions/checkout to v4 across all CI jobs

Replace inconsistent @v6 with @v4 (consistent with release-draft and release-publish workflows).

# ── git-warden policy guide ──
# Write the commit message in English
# AI co-author trailers will be removed before commit

Author: zcube

.github/workflows/ci.yml

@@ -17,7 +17,7 @@ jobs:
     name: fmt & clippy
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@stable
         with:
           components: rustfmt, clippy
@@ -35,7 +35,7 @@ jobs:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@stable
       - uses: Swatinem/rust-cache@v2
       - name: build
@@ -47,7 +47,7 @@ jobs:
     name: MSRV (1.88)
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@1.88
       - uses: Swatinem/rust-cache@v2
       - name: build