add undo-aware dirty tracking for documents

[Third-Thing]

The original proposal (#1054) resulted in users needing to subscribe to cache_rev for reactivity:

Label::derived(move || {
    let _ = doc.cache_rev().get();
    if doc.is_dirty() { "Unsaved" } else { "Saved" }.to_string()
})

This led to a choice between a dedicated reactive dirty-state API, Document::dirty() -> Memo<bool>, or making is_dirty() reactive.

The main reason for choosing the Memo approach was separation of concerns. is_dirty() remains a plain imperative query, while dirty() is the explicit reactive hook for UI. That removes the need to piggyback on cache_rev(), keeps dirty-state subscriptions decoupled from layout/cache invalidation, and makes application code clearer: views subscribe to doc.dirty().get(), while non-reactive code can still call doc.is_dirty().

Further, a tracked is_dirty() would hide reactive behavior behind what looks like a normal getter and would implicitly subscribe callers to the full buffer signal. A dedicated memo is more explicit and only propagates when the dirty boolean actually changes.

With this change, reactive UI becomes:

Label::derived(move || {
    if doc.dirty().get() { "Unsaved" } else { "Saved" }.to_string()
})

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This would be a breaking change for types that implement Document. However, given that the existing Document methods imply a dynamic document (not static) it's assumed that the ability to properly track changes will be welcome.