Fix inline LaTeX fallback rendering

[X (xshengcn)]

Summary

Fix inline LaTeX rendering in List on light backgrounds.

When inline formulas are rendered as text attachments, TextKit can display the attachment's default opaque fallback image. In a SwiftUI List, that fallback appears as a white rectangle and can persist after view reuse. This change uses a transparent fallback attachment and keeps the LaTeX label transparent, so the list background remains visible.

Fixes #171

Validation

• Built SolveMate for the iOS Simulator (iPhone 17 Pro, iOS 26.5).
• Verified inline formulas in Light Mode inside List after leaving and re-entering the screen.
• Verified ScrollView rendering remains unchanged.
• Confirmed the package and SolveMate build resolve the forked dependency successfully.

OSS readiness

• No secrets, internal URLs, private identifiers, or product-only service names were added.
• No public docs, fixtures, or notices are required for this rendering-only fix.
• Third-party dependency changes (adds, removes, version bumps) are intentional and reviewed.
• Streaming/incomplete markdown behavior remains covered by existing package tests.

[Copilot]

Pull request overview

This PR addresses an iOS/macOS TextKit rendering artifact where inline LaTeX attachments can momentarily display an opaque fallback image (notably in SwiftUI List row reuse), by providing a transparent fallback attachment image and ensuring the LaTeX label itself is non-opaque/clear so list backgrounds remain visible.

Changes:

• Introduces a custom LatexAttachment subclass that returns a transparent 1×1 fallback image instead of the default opaque fallback.
• Switches inline-LaTeX attachment creation to use LatexAttachment (so TextKit’s fallback drawing stays transparent).
• Makes the LaTeX label explicitly non-opaque on UIKit to avoid background artifacts.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
Sources/MarkdownText/UI/Paragraph/LatexViewProvider.swift	Adds LatexAttachment with transparent fallback image; ensures MTMathUILabel is non-opaque/clear on UIKit.
Sources/MarkdownText/Inline/Markdown+InlineConvertible.swift	Uses LatexAttachment when producing inline-LaTeX NSTextAttachments so TextKit fallback is transparent.

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[X (xshengcn)]

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