ui: File › New starts an empty model (file.new, #144); an unknown sample name opens the samples (#200)

file.new replaces the session with an empty one: no design, mesh, definitions, results or
journal, nothing to save, the undo history empty. The File menu and Cmd/Ctrl+N run it, asking
first over unsaved changes. A sample name the app does not know (a typo, an old ?sample= link)
opens the samples to choose from, and the status names it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/cmd/mod.rs

@@ -135,6 +135,7 @@ pub static ALL: &[CommandSpec] = &[
     define::STEP_ACTIVATE,
     define::RENAME,
     define::ACTIVITY,
+    file::NEW,
     file::OPEN,
     file::SAVE,
     file::SAVE_AS,