Fix crashes from stale text positions

[whitneyland]

Summary

• Validate text positions before indexing resolved layouts.
• Return nil or a neutral offset when UIKit asks about positions retained across a layout reset.
• Add regression coverage for a non-empty layout becoming empty.

Why

UIKit can retain UITextPosition values while SwiftUI replaces Textual's layout collection. If the new collection is empty, offset(from:to:) currently indexes layouts[0] and traps. Treating those positions as invalid lets UIKit discard the stale query safely.

Testing

• make test-ios — 161 tests passed.
• make test-macos — 139 tests passed.
• Built a downstream iOS app against the local package branch.

Fixes #72

AI tooling assisted with crash analysis and implementation. I reviewed the changes and test results.

[whitneyland]

Hi, if you have any questions I'm glad to discuss further. Great work on the project, it's really impressive.