Port selectable Markdown native renderer to Android

[HorusGoul]

Context

@t3tools/mobile-markdown-text provides the iOS-native selectable Markdown renderer. It is backed by Fabric components T3MarkdownText and T3MarkdownTextRun, with iOS code under apps/mobile/modules/t3-markdown-text/ios.

The package currently disables Android native linking in apps/mobile/modules/t3-markdown-text/react-native.config.js, and the codegen native component excludes Android.

Why this matters

The iOS implementation gives the conversation and file-preview UI selectable rich Markdown while preserving native text behavior. Android currently relies on app-level JS Markdown fallbacks in call sites such as the thread feed and Markdown file preview.

Important behavior to preserve:

• selectable assistant and user message Markdown
• links and file-link presentation
• inline code and code blocks
• headings, lists, tables, blockquotes, dividers, and images
• inline file icons and skill chips
• code block copy affordances
• native text layout and selection-change events

Feature examples

• Assistant responses in ThreadFeed should render rich Markdown and remain selectable.
• User messages with review-comment segments should keep readable Markdown formatting.
• Markdown file previews should render with native selectable text when available.

Acceptance criteria

• Add Android support for the Markdown native text primitives or an equivalent Android-native renderer.
• Match the existing exported renderer contract used by apps/mobile/src/native/SelectableMarkdownText.ios.tsx.
• Remove the Android exclusion/null behavior once the native implementation is available.
• Preserve selectable text, link press handling, inline attachment icons, and rich block rendering.
• Keep JS fallbacks safe for builds where the native view is unavailable.
• Add targeted tests for shared Markdown transformation logic plus Android-native rendering/selection behavior where practical.

Current test coverage note

There is substantial TypeScript coverage for Markdown transformation/link logic (apps/mobile/src/lib/nativeMarkdownText.test.ts, apps/mobile/src/lib/markdownLinks.test.ts) and widget-level consumers, but I did not find native Objective-C++/UIKit tests for T3MarkdownText itself or Android native tests for a corresponding renderer.