Eliminate inconsistent cursor state across document revisions

[Third-Thing]

The shared-editor crash issue (#934) exposed a broader API problem.

TextDocument currently has three edit paths:

• receive_char
• do_edit
• edit / edit_single

The first two are editor-aware. The third mutates the document directly without any Editor or shared-group context.

That leads to several problems:

• programmatic edits can leave a single editor with a stale cursor
• editors attached to the same document can keep stale cursors after another editor modifies the text
• edit / edit_single do not record cursor_before / cursor_after, so undo lacks full editor state

Proposal

Deprecate:

• Document::edit(...)
• Document::edit_single(...)

Make editor-aware editing the primary API:

• Editor::edit(...)
• EditorGroup::edit(...)

If multiple editors share one document, EditorGroup gives one place to coordinate:

• attached editors
• cursor remapping
• undo-related editor state
• future shared-edit behavior

The reported crashes, plus the editor example’s intentional suppression of undo in one shared editor, demonstrate the need for this.

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Status Update: Ready for review

[jrmoulton]

@panekj @dzhou121

[panekj]

I'm ok with making it a #[doc(hidden)] or maybe feature-gate it but that API has purpose and we use it in Lapce
Marking it as deprecated is good start to notify people to stop using it

[Third-Thing]

While implementing EditorGroup, I realized the design strayed from floem's reactive nature and required several breaking changes. That's unnecessary: The two linked PRs solve all three problems this issue originally flagged.

Something like EditorGroup or EditDirector might still be worth exploring in the future if someone wanted to work toward collaborative editing.