Fix Cmd+A highlight missing last line on trailing-newline documents

[BatchZero]

Summary

• TextLineStorageRangeIterator.next() stopped at currentPosition.range.max < range.max. When the document ends with a line ending, line storage holds a trailing empty line at the very offset where the previous line's range.max equals range.max, so the iterator never reached it.
• That meant selectAll selected the line (Cmd+C/Cmd+V worked) but TextSelectionManager.getFillRects never drew its selection rect — exactly the symptom reported in 🐞 Command-A (Copy All) is capturing, but not highlighting #121.
• The fix lets the iterator yield a length-0 line whose location equals range.max. Every other case is left unchanged, including:
  • linesInRange(NSRange(location: 6, length: 0)) (used by editingEndOfDocumentInvalidatesLastLine)
  • partial ranges that end exactly at the start of a non-empty line (linesInRange should not include that line)
  • empty lines in the middle of a document (already covered by rangeIteratorDoesNotSkipEmptyLines)

Fixes #121.

Test plan

• swift test — all suites green (44 XCTest cases + 66 swift-testing cases)
• New regression test rangeIteratorIncludesTrailingEmptyLine in TextLayoutManagerTests covers the trailing-newline scenario