Improve string replace performance

[cyanzhong]

No description provided.

[Copilot]

🟢 Approval recommended

The changes are small, consistent with the intended performance goal, and the updated test adds a concrete guardrail for the placeholder assumptions the optimization relies on.

Pull request overview

This PR targets string replacement hotspots by making replacements more explicit/literal and by avoiding unnecessary scans of large injected JSON when generating the editor’s index.html.

Changes:

• Use literal string replacement options for line-break sanitization when copying pasteboard content.
• Optimize index.html placeholder substitution by replacing the later placeholder first and limiting replacements to a single occurrence.
• Strengthen the CoreEditor build test to assert placeholder uniqueness and ordering.

File summaries

File	Description
MarkEditMac/Modules/Sources/AppKitExtensions/Foundation/NSPasteboard+Extension.swift	Forces literal replacements during line-break normalization.
MarkEditCore/Sources/EditorIndexHtml.swift	Reorders and bounds placeholder replacement to reduce scanning work.
MarkEditCore/Sources/EditorImageLoader.swift	Refactors formatting only (no behavioral change).
CoreEditor/test/build.test.ts	Validates placeholders occur once and appear in the expected order.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 0
• Review effort level: Lite

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