feat: autosave diff comments

[matt2e]

Summary:

• Add autosave support for new and existing diff comments.
• Preserve comment persistence failures and harden dismissal/navigation races.
• Add focused autosave and review state coverage.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Flush pending comments before unmounting editors

When a host changes the displayed diff without calling the new imperative flush API, this onDestroy autosave runs after the editor is already being torn down and its onSave callback in DiffViewer derives the path/span from current component state rather than the original editor context. I checked apps/differ/src/App.svelte::selectFile, which still sets selectedFile immediately and does not bind/call flushCommentEditors; typing a new comment there and immediately selecting another file before the debounce fires can drop the draft (or save against the wrong current file) instead of autosaving it. Capture the editor's original path/span for the save callback or flush from all file-switch paths before changing the diff.

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