Make MarkdownController and StreamedMarkdownController lifecycle methods async

[junyan72]

Summary

The lifecycle methods on MarkdownController and StreamedMarkdownController are currently synchronous, even though every call site invokes them from an async context (.task { ... }) or a place where a Task { } wrapper is already implied.

Marking them async lets the call sites await them directly, aligns the controllers with the package's Swift-concurrency conventions (AGENTS.md), and keeps the async boundary explicit at the view layer.

Methods to convert:

• StreamedMarkdownController.start() / end()
• MarkdownController.onAppear(markdown:) / onDisappear()

The fire-and-forget interaction hooks (onTableCopyTap, onImageTap, etc.) and the sync onChange(markdown:) yield-into-continuation method are intentionally left synchronous, since their call sites are non-async UIKit/AppKit delegates and SwiftUI action closures.

Motivation

• Call sites already run inside .task { } (an async context), so await-ing is natural and avoids implicit fire-and-forget.
• Makes the concurrency boundary explicit and consistent across the two controllers.