Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Closes #56748

## Summary

Block / column selection (Shift+Ctrl+drag on Linux, Shift+Cmd+drag on
macOS) anchored its rectangle on a single byte column applied to every
row in the dragged range. On a row whose text contained a multi-byte
character before that column (for example `ã`), the same byte offset
mapped to a different visual `x`, so the per-row cursor drifted off the
column.

`select_columns` now anchors the rectangle in `x` pixels via
`x_for_display_point` and resolves each row's byte column with
`display_column_for_x` — the same pair of helpers already used by
vertical cursor motion and Vim VisualBlock.

## Before / After

**Before** — cursor on the `ã` row drifts one column to the left:

<img width="1068" height="366" alt="image"
src="https://github.com/user-attachments/assets/0559ef89-852e-40f3-9d43-f5f44277f99e"
/>


**After** — rectangle stays visually aligned across rows:

<img width="866" height="408" alt="image"
src="https://github.com/user-attachments/assets/bce13c1a-1ddb-4fd6-8216-2c0e902f6668"
/>


## Test plan

- `cargo test -p editor test_columnar_selection_with_multibyte_chars`
(new regression test covers the buggy case past the `ã` byte boundary,
plus a control case where old and new logic agree).
- Manual: Shift+Ctrl+drag down a five-line buffer where row 3 contains
`ã` — selection rectangle stays aligned (see screenshots).
- Manual: Shift+Alt+drag (`FromSelection` branch) over the same buffer —
same result.
- Manual: drag past EOL on the `ã` row — each row extends to its own EOL
with no panic.

Release Notes:

- Fixed misalignment of column selection on rows that contain multi-byte
characters

---------

Co-authored-by: Tom Houlé <tom@tomhoule.com>