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/autocorrect-replacement-lines.ts

@@ -15,6 +15,7 @@ export function stripMergeOperatorChars(text: string): string {
 }
 
 function leadingWhitespace(text: string): string {
+  if (!text) return ""
   const match = text.match(/^\s*/)
   return match ? match[0] : ""
 }
@@ -36,7 +37,9 @@ export function restoreOldWrappedLines(originalLines: string[], replacementLines
   const candidates: { start: number; len: number; replacement: string; canonical: string }[] = []
   for (let start = 0; start < replacementLines.length; start += 1) {
     for (let len = 2; len <= 10 && start + len <= replacementLines.length; len += 1) {
-      const canonicalSpan = stripAllWhitespace(replacementLines.slice(start, start + len).join(""))
+      const span = replacementLines.slice(start, start + len)
+      if (span.some((line) => line.trim().length === 0)) continue
+      const canonicalSpan = stripAllWhitespace(span.join(""))
       const original = canonicalToOriginal.get(canonicalSpan)
       if (original && original.count === 1 && canonicalSpan.length >= 6) {
         candidates.push({ start, len, replacement: original.line, canonical: canonicalSpan })

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

src/tools/hashline-edit/edit-operations.test.ts

@@ -236,6 +236,22 @@ describe("hashline edit operations", () => {
     expect(result).toEqual(["if (x) {", "  return 3", "  return 4", "}"])
   })
 
+  it("preserves blank lines and indentation in range replace (no false unwrap)", () => {
+    //#given — reproduces the 애국가 bug where blank+indented lines collapse
+    const lines = ["", "동해물과 백두산이 마르고 닳도록", "하느님이 보우하사 우리나라 만세", "", "무궁화 삼천리 화려강산", "대한사람 대한으로 길이 보전하세", ""]
+
+    //#when — replace the range with indented version (blank lines preserved)
+    const result = applyReplaceLines(
+      lines,
+      anchorFor(lines, 1),
+      anchorFor(lines, 7),
+      ["", "  동해물과 백두산이 마르고 닳도록", "  하느님이 보우하사 우리나라 만세", "", "  무궁화 삼천리 화려강산", "  대한사람 대한으로 길이 보전하세", ""]
+    )
+
+    //#then — all 7 lines preserved with indentation, not collapsed to 3
+    expect(result).toEqual(["", "  동해물과 백두산이 마르고 닳도록", "  하느님이 보우하사 우리나라 만세", "", "  무궁화 삼천리 화려강산", "  대한사람 대한으로 길이 보전하세", ""])
+  })
+
   it("collapses wrapped replacement span back to unique original single line", () => {
     //#given
     const lines = [
@@ -353,4 +369,33 @@ describe("hashline edit operations", () => {
     //#then
     expect(result).toEqual(["const a = 10;", "const b = 20;"])
   })
+
+  it("throws on overlapping range edits", () => {
+    //#given
+    const content = "line 1\nline 2\nline 3\nline 4\nline 5"
+    const lines = content.split("\n")
+    const edits: HashlineEdit[] = [
+      { op: "replace", pos: anchorFor(lines, 1), end: anchorFor(lines, 3), lines: "replaced A" },
+      { op: "replace", pos: anchorFor(lines, 2), end: anchorFor(lines, 4), lines: "replaced B" },
+    ]
+
+    //#when / #then
+    expect(() => applyHashlineEdits(content, edits)).toThrow(/overlapping/i)
+  })
+
+  it("allows non-overlapping range edits", () => {
+    //#given
+    const content = "line 1\nline 2\nline 3\nline 4\nline 5"
+    const lines = content.split("\n")
+    const edits: HashlineEdit[] = [
+      { op: "replace", pos: anchorFor(lines, 1), end: anchorFor(lines, 2), lines: "replaced A" },
+      { op: "replace", pos: anchorFor(lines, 4), end: anchorFor(lines, 5), lines: "replaced B" },
+    ]
+
+    //#when
+    const result = applyHashlineEdits(content, edits)
+
+    //#then
+    expect(result).toEqual("replaced A\nline 3\nreplaced B")
+  })
 })

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": {

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
+}

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] : ""
 }

src/tools/hashline-edit/tool-description.ts

@@ -34,8 +34,8 @@ FILE CREATION:
   CRITICAL: only unanchored append/prepend can create a missing file.
 
 OPERATION CHOICE:
-  replace with pos only -> replace one line at pos
-  replace with pos+end -> replace range pos..end
+  replace with pos only -> replace one line at pos (MOST COMMON for single-line edits)
+  replace with pos+end -> replace ENTIRE range pos..end as a block (ranges MUST NOT overlap across edits)
   append with pos/end anchor -> insert after that anchor
   prepend with pos/end anchor -> insert before that anchor
   append/prepend without anchors -> EOF/BOF insertion