fix(input): handle panelless direct commits reliably

[Tsingv]

Summary

• defer panelless direct commits to the next main-runloop turn for the VS Code integrated terminal
• track and clear synthetic marked text so clients such as Alacritty do not consume the first unhandled key after punctuation
• preserve normal synchronous commits when a real preedit is active
• configure the workarounds only for the known affected applications

Motivation

This is a follow-up to #1165.

VS Code's integrated terminal (xterm.js) can drop insertText when a panelless commit, such as full-width punctuation, is delivered synchronously during keyDown. Alacritty, on the other hand, needs the synthetic marked-text phase but may retain it after insertText, causing the first Backspace to cancel composition instead of deleting text.

The two behaviors are therefore kept independently configurable:

• force_marked_text_for_direct_commit synthesizes the marked-text phase
• defer_direct_commit moves the final insertion to the next main-runloop turn

Default behavior

As discussed in #1165, I also tested enabling the marked-text workaround globally. The earlier implementation caused font fallback in Microsoft Word. The cleanup in this PR has fixed that issue in current testing, but broader testing is still in progress.

For now, this PR does not make the workaround global. It remains scoped through app_options to the known affected clients: Alacritty and VS Code.

Testing

• tested Chinese input and full-width punctuation in Alacritty
• verified that the first Backspace after a direct punctuation commit is no longer swallowed
• tested Chinese input and full-width punctuation in the VS Code integrated terminal
• retested Microsoft Word, including the previously observed font-fallback case
• built the Debug target with CODE_SIGNING_ALLOWED=NO (BUILD SUCCEEDED)

[lotem]

我反對針對客戶程序的BUG做特殊處理。
軟件邏輯不考慮兼容輸入法，那就不要爲了強行兼容，在輸入法代碼裏加入難以理解、難以維護的補丁。

[lotem]

比起 #1172 還是這個實現可靠一些。
我再看看……

[lotem]

兩個 commit 分開看，能接受……
可是這類補丁打多了，代碼難免成麪條。

爲解 BUG 可以先收着。
@LEOYoon-Tsaw 有意見沒？