Fix undo/redo corruption when typing at the end of a final-newline file

[user77 (hexbinoct)]

Fixes #834

What breaks

Typing at the end of a buffer that has the automatic POSIX final newline enabled (the default for new files on non-Windows, and enabled on any platform when an opened file ends in a newline), then pressing Ctrl+Z and Ctrl+Y, corrupts the text: ab plus trailing newline comes back as a, newline, b. Deterministic repro and full analysis are on the issue.

Root cause

The insert_final_newline branch at the end of TextBuffer::write() records the appended newline in the current undo entry and then moves the cursor back before it by assigning self.cursor directly. That bypasses the coalescing break in set_cursor(), so the next typed character coalesces into the same undo entry. Its bytes land before the final newline in the buffer while the entry's added vector records them after it: buffer ab\n, recorded a\nb. Undo deletes by byte count and works; redo replays added as one contiguous string and reproduces write order.

Fix

Stop coalescing after a final-newline insertion (self.last_history_type = HistoryType::Other; in that branch), so every undo entry's recorded bytes stay contiguous in buffer order. The cost is one extra undo step when typing begins at the end of such a file, since the first character and its automatic newline form their own entry.

Tests

crates/edit/tests/undo_redo_834.rs adds six round-trip tests: coalesced typing with the final newline (LF and CRLF), a double undo/redo cycle, a single multi-line write (LF and CRLF), and coalesced typing across a newline without the final-newline feature. The three final-newline tests fail on main and pass with this change. The multi-line write tests pass on main as well as here; they are included to pin down that the redo reinsert loop was not the problem (relevant to #852, where I left verification details).

Full workspace test suite and clippy pass locally on Windows; the fix itself is one line.