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-operation-primitives.ts

@@ -63,7 +63,7 @@ export function applyReplaceLines(
   const corrected = autocorrectReplacementLines(originalRange, stripped)
   const restored = corrected.map((entry, idx) => {
     if (idx !== 0) return entry
-    return restoreLeadingIndent(lines[startLine - 1], entry)
+    return restoreLeadingIndent(lines[startLine - 1] ?? "", entry)
   })
   result.splice(startLine - 1, endLine - startLine + 1, ...restored)
   return result