ci: move full WSL2 suite to nightly, keep per-PR contract gate

- the full serial suite needs 50+ minutes on the 9P filesystem
  (2026/2343 lib tests finished in 39.5min before the 45min timeout)
- per-PR job now runs only the atomic-write contract (~6min total)
  and builds just the lib test binary via --lib
- new nightly workflow (schedule + workflow_dispatch, 90min timeout)
  runs the contract plus the full serial suite on the WSL2 home

.github/workflows/ci.yml

@@ -157,7 +157,7 @@ jobs:
 
       - name: Compile backend tests with native temp
         # link.exe/mt.exe cannot create manifests when TEMP points at a WSL UNC path.
-        run: cargo test --tests --manifest-path src-tauri/Cargo.toml --no-run
+        run: cargo test --lib --manifest-path src-tauri/Cargo.toml --no-run
 
       - name: Run Windows-to-WSL2 filesystem contract
         shell: pwsh
@@ -165,7 +165,7 @@ jobs:
           $env:TEMP = $env:CC_SWITCH_WSL_TEST_TEMP
           $env:TMP = $env:CC_SWITCH_WSL_TEST_TEMP
           $testName = "config::tests::atomic_write_replaces_existing_wsl_unc_file"
-          $testList = cargo test --tests --manifest-path src-tauri/Cargo.toml -- --ignored --list
+          $testList = cargo test --lib --manifest-path src-tauri/Cargo.toml -- --ignored --list
           if ($LASTEXITCODE -ne 0) {
             exit $LASTEXITCODE
           }
@@ -173,14 +173,7 @@ jobs:
           if ($testList -notcontains $expected) {
             throw "Expected ignored test was not discovered: $testName"
           }
-          cargo test --tests --manifest-path src-tauri/Cargo.toml $testName -- --ignored --exact --nocapture
+          cargo test --lib --manifest-path src-tauri/Cargo.toml $testName -- --ignored --exact --nocapture
           if ($LASTEXITCODE -ne 0) {
             exit $LASTEXITCODE
           }
-
-      - name: Run tests against WSL2-backed home
-        shell: pwsh
-        run: |
-          $env:TEMP = $env:CC_SWITCH_WSL_TEST_TEMP
-          $env:TMP = $env:CC_SWITCH_WSL_TEST_TEMP
-          cargo test --tests --manifest-path src-tauri/Cargo.toml -- --test-threads=1

.github/workflows/wsl2-nightly.yml

@@ -0,0 +1,89 @@
+name: WSL2 Nightly
+
+# The full backend suite takes 50+ minutes on a \\wsl.localhost 9P filesystem,
+# so it runs nightly instead of per PR. The per-PR gate in ci.yml only runs the
+# atomic-write contract test against the same WSL2-backed environment.
+on:
+  schedule:
+    - cron: "23 18 * * *" # 02:23 Beijing time
+  workflow_dispatch:
+
+jobs:
+  backend-windows-wsl2-full:
+    name: Backend Full Suite (Windows + WSL2 home)
+    runs-on: windows-2025
+    timeout-minutes: 90
+    permissions:
+      contents: read
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+
+      - name: Setup Rust
+        uses: dtolnay/rust-toolchain@4360b52568e2003a75bf9bc1d59f33a8e3fc893c # stable
+
+      - name: Setup WSL2
+        uses: Vampire/setup-wsl@d1da7f2c0322a5ee4f24975344f67fc0f5baf364 # v7
+        with:
+          distribution: Ubuntu-24.04
+          wsl-version: 2
+
+      - name: Cache Cargo registry and build
+        uses: actions/cache@v5
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            src-tauri/target
+          key: ${{ runner.os }}-cargo-${{ hashFiles('src-tauri/Cargo.lock') }}
+          restore-keys: ${{ runner.os }}-cargo-
+
+      - name: Create frontend dist placeholder
+        shell: bash
+        run: mkdir -p dist
+
+      - name: Prepare WSL2-backed test environment
+        shell: pwsh
+        run: |
+          wsl.exe -d Ubuntu-24.04 -- bash -lc "rm -rf /tmp/cc-switch-windows-test-root && mkdir -p /tmp/cc-switch-windows-test-root/{home,temp} && chmod -R 0777 /tmp/cc-switch-windows-test-root"
+          $testRoot = "\\wsl.localhost\Ubuntu-24.04\tmp\cc-switch-windows-test-root"
+          $testHome = Join-Path $testRoot "home"
+          $testTemp = Join-Path $testRoot "temp"
+          foreach ($path in @($testRoot, $testHome, $testTemp)) {
+            if (-not (Test-Path -LiteralPath $path)) {
+              throw "WSL2-backed test path is unavailable: $path"
+            }
+          }
+          "CC_SWITCH_TEST_HOME=$testHome" | Out-File -FilePath $env:GITHUB_ENV -Encoding utf8 -Append
+          "CC_SWITCH_WSL_TEST_DIR=$testRoot" | Out-File -FilePath $env:GITHUB_ENV -Encoding utf8 -Append
+          "CC_SWITCH_WSL_TEST_TEMP=$testTemp" | Out-File -FilePath $env:GITHUB_ENV -Encoding utf8 -Append
+
+      - name: Compile backend tests with native temp
+        # link.exe/mt.exe cannot create manifests when TEMP points at a WSL UNC path.
+        run: cargo test --tests --manifest-path src-tauri/Cargo.toml --no-run
+
+      - name: Run Windows-to-WSL2 filesystem contract
+        shell: pwsh
+        run: |
+          $env:TEMP = $env:CC_SWITCH_WSL_TEST_TEMP
+          $env:TMP = $env:CC_SWITCH_WSL_TEST_TEMP
+          $testName = "config::tests::atomic_write_replaces_existing_wsl_unc_file"
+          $testList = cargo test --tests --manifest-path src-tauri/Cargo.toml -- --ignored --list
+          if ($LASTEXITCODE -ne 0) {
+            exit $LASTEXITCODE
+          }
+          $expected = "${testName}: test"
+          if ($testList -notcontains $expected) {
+            throw "Expected ignored test was not discovered: $testName"
+          }
+          cargo test --tests --manifest-path src-tauri/Cargo.toml $testName -- --ignored --exact --nocapture
+          if ($LASTEXITCODE -ne 0) {
+            exit $LASTEXITCODE
+          }
+
+      - name: Run full test suite against WSL2-backed home
+        shell: pwsh
+        run: |
+          $env:TEMP = $env:CC_SWITCH_WSL_TEST_TEMP
+          $env:TMP = $env:CC_SWITCH_WSL_TEST_TEMP
+          cargo test --tests --manifest-path src-tauri/Cargo.toml -- --test-threads=1