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-ordering.ts

@@ -27,3 +27,30 @@ export function collectLineRefs(edits: HashlineEdit[]): string[] {
     }
   })
 }
+
+export function detectOverlappingRanges(edits: HashlineEdit[]): string | null {
+  const ranges: { start: number; end: number; idx: number }[] = []
+  for (let i = 0; i < edits.length; i++) {
+    const edit = edits[i]
+    if (edit.op !== "replace" || !edit.end) continue
+    const start = parseLineRef(edit.pos).line
+    const end = parseLineRef(edit.end).line
+    ranges.push({ start, end, idx: i })
+  }
+  if (ranges.length < 2) return null
+
+  ranges.sort((a, b) => a.start - b.start || a.end - b.end)
+  for (let i = 1; i < ranges.length; i++) {
+    const prev = ranges[i - 1]
+    const curr = ranges[i]
+    if (curr.start <= prev.end) {
+      return (
+        `Overlapping range edits detected: ` +
+        `edit ${prev.idx + 1} (lines ${prev.start}-${prev.end}) overlaps with ` +
+        `edit ${curr.idx + 1} (lines ${curr.start}-${curr.end}). ` +
+        `Use pos-only replace for single-line edits.`
+      )
+    }
+  }
+  return null
+}