Build & attach Windows prebuilt binary to v0.2.0-alpha.1

[Fanaperana]

Per the README install matrix, we're missing a Windows tarball for v0.2.0-alpha.1.

Target needed:

• x86_64-pc-windows-msvc

Notes

• The Windows build does not include Tesseract OCR (see the README platform-support matrix). Users should rely on --ocr-server-url for OCR on Windows, or compile locally with --features spdf-cli/tesseract after installing Tesseract via vcpkg.
• build.rs downloads a Windows-x64 pdfium from bblanchon/pdfium-binaries automatically.

Recipe (PowerShell, on a Windows x64 host with Rust 1.85+):

git checkout v0.2.0-alpha.1
cargo build --release -p spdf-cli
$VER = "0.2.0-alpha.1"
$TARGET = "x86_64-pc-windows-msvc"
$DIR = "spdf-$VER-$TARGET"
New-Item -ItemType Directory -Force -Path "dist\$DIR" | Out-Null
Copy-Item target\release\spdf.exe         "dist\$DIR\"
Copy-Item LICENSE,README.md,CHANGELOG.md  "dist\$DIR\"
Compress-Archive -Path "dist\$DIR" -DestinationPath "dist\$DIR.zip"
(Get-FileHash "dist\$DIR.zip" -Algorithm SHA256).Hash | Out-File "dist\$DIR.zip.sha256"
gh release upload v$VER "dist\$DIR.zip" "dist\$DIR.zip.sha256"

After uploading, check the target off in the release notes body and the README matrix.