Add AppKit NSTextDiffView API and unify SwiftUI rendering

[iSapozhnik]

Summary

• add public NSTextDiffView with API parity to SwiftUI TextDiffView (original, updated, style, mode)
• move AppKit drawing and diff-update lifecycle into NSTextDiffView with guarded recompute for unchanged inputs
• refactor TextDiffView/DiffTextViewRepresentable to use NSTextDiffView as the shared rendering path
• split DiffTextLayoutMetrics into its own file and remove old DiffCanvasView
• update README with AppKit usage, update-in-place example, sizing notes, and snapshot docs

Tests

• swift test 2>&1 | xcsift

Added coverage

• new behavior tests for NSTextDiffView updates/recompute semantics
• new direct AppKit snapshot suite and baselines under Tests/TextDiffTests/__Snapshots__/NSTextDiffSnapshotTests/