Fix NSRangeException crash in updateCodeBlockSelection after document swap

[wishworldbetter]

Crash

A macOS app embedding this package crashes with SIGABRT via an uncaught NSRangeException from -[NSString substringWithRange:]. Faulting stack from a real .ips crash report:

Foundation  -[NSString substringWithRange:]
mm          closure #2 in NativeTextViewCoordinator.updateCodeBlockSelection(textView:parsed:)
libswiftCore Sequence.compactMap
mm          NativeTextViewCoordinator.updateCodeBlockSelection(textView:parsed:)
mm          closure #7 in NativeTextViewWrapper.updateNSView(_:context:)   (dispatched async to main)

Root cause

In Sources/MarkdownEngine/TextView/Coordinator/NativeTextViewCoordinator+CodeBlocks.swift, updateCodeBlockSelection reads cachedCodeBlockTokens — token ranges produced by a previous parse — and calls nsText.substring(with: token.contentRange) against the text view's current string.

When the host swaps documents and the new text is shorter than the old one, an async-queued update from updateNSView can run with the stale cached tokens. Their ranges exceed the new string's length, so substringWithRange: raises NSRangeException and the process aborts. Any document-switch path can trigger it.

Reproduction sketch

1. Embed the editor in a host app.
2. Open a document that has a fenced code block near its end.
3. Switch to a shorter document. The queued updateCodeBlockSelection call still holds the old parse's token cache, whose ranges are out of bounds for the new string → crash.

Fix

Add a bounds guard at the top of the compactMap closure that skips tokens whose range or contentRange extends past the current string length. Stale tokens are only possible for one async hop after a document swap; the next parse refreshes the cache and the copy-button overlays come back with correct geometry.

🤖 Generated with Claude Code

[luca-chen198]

lgtm