fix(hashline-edit): detect overlapping ranges and prevent false unwrap of blank-line spans

- Add detectOverlappingRanges() to reject edits with overlapping pos..end ranges
  instead of crashing with undefined.match()
- Add bounds guard (?? "") in edit-operation-primitives for out-of-range line access
- Add null guard in leadingWhitespace() for undefined/empty input
- Fix restoreOldWrappedLines false unwrap: skip candidate spans containing
  blank/whitespace-only lines, preventing incorrect collapse of structural
  blank lines and indentation (the "애국가 bug")
- Improve tool description for range replace clarity
- Add tests: overlapping range detection, false unwrap prevention

src/tools/hashline-edit/edit-text-normalization.ts

@@ -7,6 +7,7 @@ function equalsIgnoringWhitespace(a: string, b: string): boolean {
 }
 
 function leadingWhitespace(text: string): string {
+  if (!text) return ""
   const match = text.match(/^\s*/)
   return match ? match[0] : ""
 }