[srudraDev]

Opening another workbook from an existing native GridCraft window now launches
it in a separate native window, preserving the current workbook.

• Add a native open_in_new_window host service for desktop shells.
• Validate workbook files before launching a new process so malformed or
  unreadable files report an error in the current UI.
• Prevent duplicate windows for already-open workbooks, including equivalent
  relative, absolute, and canonical paths.
• Preserve the initial blank-workbook replacement behavior.
• Keep CLI, web, headless, and test behavior unchanged when no native window
  service is provided.
• Surface native launch failures without modifying the current workbook.

Validation

• cargo fmt --all
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test -p gridcraft-ui-egui
• cargo check -p gridcraft
• cargo check -p gridcraft-web --target wasm32-unknown-unknown
• git diff --check

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[Zarvinx]

Hey, nice work! I somehow missed this before starting #171, so we’ve ended up exploring two different approaches to the same problem. Sorry about the duplicate work.

I like how small and straightforward this implementation is. It gives each workbook a real native window by launching a separate GridCraft process.

In #171 I took the other route: multiple native egui windows coordinated inside one process. That adds more lifecycle work, but it means GridCraft can:

• focus an existing workbook window from the selector
• route Finder drops and Cmd+N/Cmd+O through one coordinator
• focus the existing window when the same file is opened again
• share preferences, theme and recent files between windows
• stay running on macOS after the last workbook window closes