Improve Windows workflow: add Rust toolchain setup, caching, and better diagnostics

Co-authored-by: jgarzik <[email protected]>

Author: Copilot

.github/workflows/windows-build.yml

@@ -12,13 +12,18 @@ jobs:
     runs-on: windows-latest
     steps:
       - uses: actions/checkout@v4
+      - name: Setup Rust toolchain
+        uses: dtolnay/rust-toolchain@stable
+      - name: Setup Rust cache
+        uses: Swatinem/rust-cache@v2
       - name: System info
         run: |
           echo "=== Architecture ==="
           systeminfo | findstr /B /C:"OS Name" /C:"OS Version" /C:"System Type"
           echo "=== Toolchain ==="
           rustc --version
           cargo --version
+          cl /? 2>&1 | findstr /C:"Version" || echo "MSVC not in PATH"
       - name: Fetch
         run: cargo fetch
       - name: Build