fix(hashline): address Cubic review comments

- P2: Change replace edit sorting from POSITIVE_INFINITY to NEGATIVE_INFINITY
  so replace edits run LAST after line-based edits, preventing line number
  shifts that would invalidate subsequent anchors

- P3: Update tool description from SHA-256 to xxHash32 to match actual
  implementation in hash-computation.ts

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

@@ -62,7 +62,7 @@ function getEditLineNumber(edit: HashlineEdit): number {
     case "insert_after":
       return parseLineRef(edit.line).line
     case "replace":
-      return Number.POSITIVE_INFINITY
+      return Number.NEGATIVE_INFINITY
     default:
       return Number.POSITIVE_INFINITY
   }