The composable test interpreter wired its own copy of the undo stack
subscription and started only the component watcher, so it reacted to
edits differently from the app, and a missing subscription failed
silently.
Gather the subscriptions that react to edits while a file is open
(undo stack, layout updates, text reflow, WASM text measurement) in
initialize-edit-watchers, stopped by finalize-edit-watchers, and the
per-page component and token watchers in watch-library-changes. The
workspace and the interpreter both start them.
Starting the edit watchers again replaces the running ones. A file
reload starts the workspace again without finalizing it, which left two
undo stack watchers running, so an edit added an extra undo entry.
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