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/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