feat(ci): cross-compile Windows binaries from Linux using cargo-xwin

Summary

Replace the windows-latest GitHub Actions runner for binary builds with cargo-xwin cross-compilation from Linux. This eliminates the need for a Windows runner entirely, reducing CI cost and simplifying runner management.

Background

The current release workflow (release-comprehensive.yml) uses a windows-latest runner to build x86_64-pc-windows-msvc binaries natively. This works but:

• Windows runners cost ~2x more than Linux ($0.016/min vs $0.008/min)
• Adds a separate OS to maintain in CI matrix
• Has caused packaging issues (e.g., #559 -- zip not available on Windows runners, fixed by #565 with 7z)

Proposed Change

Use cargo-xwin on a Linux runner to cross-compile Windows MSVC binaries:

# Replace windows-latest matrix entry with:
- os: ubuntu-22.04
  target: x86_64-pc-windows-msvc
  use_cross: false
  cross_tool: cargo-xwin

Prerequisites on Linux runner

rustup target add x86_64-pc-windows-msvc
cargo install cargo-xwin
sudo apt-get install -y clang llvm

Build command

cargo xwin build --release --target x86_64-pc-windows-msvc

Dependency compatibility

• reqwest (rustls-tls): Pure Rust TLS -- no OpenSSL needed, works seamlessly
• tokio: Full support via IOCP on Windows target
• salvo: Standard async framework, no platform-specific issues
• serde, clap, ratatui, crossterm: All cross-platform compatible

Scope

• Add cargo-xwin installation step to release workflow
• Replace windows-latest matrix entry with Linux + cargo-xwin for binary builds
• Keep windows-latest for Tauri desktop builds (requires native Windows SDK)
• Update artifact packaging (use standard zip on Linux instead of 7z)
• Cache Windows SDK download (cargo xwin cache in v0.21.4+)
• Validate produced .exe binaries work correctly

Notes

• Tauri desktop builds (build-tauri-desktop) must stay on windows-latest since they need the full Windows SDK for MSI/NSIS installer generation
• Only the CLI binary builds (build-binaries matrix) should move to cargo-xwin
• The terraphim_server/build.rs has a #[cfg(target_os = "windows")] check for frontend assets that may need adjustment for cross-compilation (build.rs runs on host, not target)

Start with release-comprehensive.yml and compare the build-binaries matrix with build-tauri-desktop. Review the existing Windows packaging steps, then run the proposed cargo-xwin build on Linux and inspect terraphim_server/build.rs for the noted target check. Done means CLI Windows binaries build and package successfully through Linux while Tauri builds remain on windows-latest.