Korean IME: Backspace during Hangul composition fails to delete the last remaining jamo

[shp7724]

Issue Description

When composing a Hangul syllable with the macOS built-in 2-Set Korean IME, pressing
Backspace decomposes the syllable jamo-by-jamo as expected — until only the initial
consonant remains. The final Backspace that should delete that last jamo is ignored,
leaving the character stuck in the preedit.

This happens directly at the shell prompt as well as inside TUI apps, so it is not
app-specific. This machine reproduces it on the latest tip; my previous machine on an
older tip build does not (see regression range below).

Expected Behavior

Example: type 안 (jamo sequence ㅇ + ㅏ + ㄴ), then press Backspace 3 times:

• Expected: 안 → 아 → ㅇ → (empty)

Actual Behavior

• Actual: 안 → 아 → ㅇ → ㅇ (last Backspace is ignored; the initial consonant is not deleted)

Reproduction Steps

2026-07-24.8.08.06.mov

Ghostty Logs

No response

Ghostty Version

### Regression Range

- **Good:** `1.3.2-main+dcc39dcd4` (commit `dcc39dcd4019`, 2026-04-19) — Backspace fully decomposes the syllable, last jamo deletes fine.
- **Bad:** `1.3.2-main+b14d92383` (commit `b14d9238366f`, 2026-07-08) — last jamo cannot be deleted. Still reproduces on latest main.

OS Version Information

macOS 26.5

(Linux only) Display Server

None

(Linux only) Desktop Environment/Window Manager

No response

Minimal Ghostty Configuration

Reproducible with empty configuration

Additional Relevant Configuration

No response

I acknowledge that:

• I have reviewed the FAQ and confirm that my issue is NOT among them.
• I have searched the Ghostty repository (both open and closed Discussions and Issues) and confirm this is not a duplicate of an existing issue or discussion.
• I have checked the "Preview" tab on all text fields to ensure that everything looks right, and have wrapped all configuration and code in code blocks with a group of three backticks (```) on separate lines.

[aca]

이슈들을 모아서 해결해보려합니다. #13750

[bo2themax]

#14797