Adds a `stubDesktop` fixture that switches the browser-mode workbench to
desktop mode over a stubbed IPC layer: tests answer the commands they need,
push Channel messages and IdeEvents, and assert on the commands invoked.
Unstubbed commands fail as unavailable; documents keep the in-memory backend.
New flows covered:
- editing: modified marker and ⌘S save, ⌘R Replace All with one undo step
- code intelligence: completion, ⌘B to another file, Find Usages, in-place
rename, diagnostics with an ⌥⏎ quick fix
- VCS: ⌘K commit with message validation and default inclusion
- run: ⌃R output streaming and exit, ⌘F2 stop
- debug: breakpoint, stop with frames and variables, step over, resume
- search: Find in Files results opening at the match line; the Search
Everywhere file case no longer needs fixme
- terminal: typed command reaches the shell and its output renders
- project: Close Project to Welcome, trust prompt, open a recent project
