Build crashes on files a plugin adds to event.files after the prepare phase

Program.build() ends by undoing edits on every file it built:

for (const file of event.files) {
    file.editor.undoAll();
}

Editors are assigned during prepare(), so any file a plugin pushes onto event.files after that point has no editor, and the build throws TypeError: Cannot read properties of undefined (reading 'undoAll').

This is reachable from a supported hook. afterPrepareProgram receives event.files and is documented as mutable, but a file added there misses editor assignment and crashes the build it was added to.

Where I hit it: rooibos generates its code-coverage component from data collected during prepareFile, so it can't be registered any earlier than afterPrepareProgram. Working around it by assigning an editor ourselves:

const file = program.setFile(entry, contents);
file.editor ??= new Editor();

That works, but it depends on a build-internal detail that plugins shouldn't need to know about.

Suggested fix — either guard the cleanup loop:

file.editor?.undoAll();

or assign an editor in the same loop that serializes, so late additions are covered. The guard alone is enough to stop the crash.

Seen on 1.0.0-alpha.53; the cleanup loop is unguarded on master as well.

Start in Program.build(), especially the cleanup loop that calls undoAll() on event.files after serialization. Reproduce the issue by adding a file through the documented afterPrepareProgram hook, then verify that the build completes without a TypeError. Done means late-added files no longer crash cleanup.