Make controller lifecycle methods async

[junyan72]

Summary

Mark the lifecycle methods on StreamedMarkdownController and MarkdownController as async so their call sites — all of which run inside .task { } — can await them directly instead of relying on implicit fire-and-forget. This keeps the concurrency boundary explicit at the view layer and aligns both controllers with the package's Swift-concurrency conventions.

• StreamedMarkdownController.start() / end() are now async
• MarkdownController.onAppear(markdown:) / onDisappear() are now async
• .task call sites now await; the sync .onDisappear callbacks wrap the call in a Task

The fire-and-forget interaction hooks (onTableCopyTap, onTableDownloadTap, onContextMenuAppear, onContextMenuTap, onImageTap) and the sync onChange(markdown:) (which only yields into a continuation) intentionally remain synchronous — their call sites are non-async UIKit/AppKit delegate methods and SwiftUI action closures, so making them async would only push a Task { } wrapper out to every call site.

Refs #144

Validation

• make lint — 0 violations
• make test — all tests pass
• make build-sample — BUILD SUCCEEDED

OSS readiness

• No secrets, internal URLs, private identifiers, or product-only service names were added.
• Public docs, fixtures, or notices were updated if behavior or dependencies changed. (no public API/doc changes; methods are internal)
• Third-party dependency changes (adds, removes, version bumps) are intentional and reviewed. (no dependency changes)
• Streaming/incomplete markdown behavior remains covered by fixtures or tests.

[Copilot]

Pull request overview

This PR updates controller lifecycle APIs to be async so SwiftUI view-layer .task {} call sites can await them directly, making the concurrency boundary explicit and consistent across DocumentView and StreamedMarkdownView.

Changes:

• Made MarkdownController.onAppear(markdown:) / onDisappear() async and updated DocumentView to await/wrap calls appropriately.
• Made StreamedMarkdownController.start() / end() async and updated StreamedMarkdownView call sites accordingly.
• Introduced a debug print in MarkdownController.onChange(markdown:) (should be removed before merge).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
Sources/MarkdownText/Utilities/MarkdownListener.swift	Converts MarkdownController lifecycle methods to async; currently includes an unintended debug log in onChange.
Sources/MarkdownText/UI/DocumentView.swift	Awaits controller lifecycle methods from .task and wraps async cleanup in .onDisappear.
Sources/MarkdownText/StreamedMarkdownView.swift	Awaits streamed controller lifecycle methods and wraps async cleanup in .onDisappear.