ci: parallelize validation workflows

[sean-parent]

Summary

• Split desktop CI into an independent job and add a Windows Pester test for the PR worktree helper.
• Run the docs workflow on pull requests, deny rustdoc warnings, and avoid deploying Pages except on pushes.
• Scope workflow concurrency by ref and cancel superseded runs.

Test plan

• cargo fmt --all -- --check
• cargo build --workspace (zero warnings)
• cargo test --workspace / cargo test --doc --workspace
• cargo clippy (workspace, begin with and without default features, ez-adam with and without default features)
• RUSTDOCFLAGS='-D warnings' cargo doc --lib --no-deps --workspace

Generated with GitHub Copilot CLI