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-operations.ts

@@ -1,5 +1,5 @@
 import { dedupeEdits } from "./edit-deduplication"
-import { collectLineRefs, getEditLineNumber } from "./edit-ordering"
+import { collectLineRefs, detectOverlappingRanges, getEditLineNumber } from "./edit-ordering"
 import type { HashlineEdit } from "./types"
 import {
   applyAppend,
@@ -36,6 +36,9 @@ export function applyHashlineEditsWithReport(content: string, edits: HashlineEdi
   const refs = collectLineRefs(sortedEdits)
   validateLineRefs(lines, refs)
 
+  const overlapError = detectOverlappingRanges(sortedEdits)
+  if (overlapError) throw new Error(overlapError)
+
   for (const edit of sortedEdits) {
     switch (edit.op) {
       case "replace": {