undofile persistence boundary with writebackup on read-only buffers
Persistent undo via 'undofile' and 'undodir' is a documented Vim feature for retaining undo history across sessions independent of swap-based crash recovery. The documented boundary between undo persistence and file write semantics is relevant when a buffer is read-only and 'writebackup' is configured. The interaction of 'undofile' with write access checks,
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