fix(hashline-edit): address Cubic review issues - boundary echo, chunking dedup, empty stream alignment

- Fix single-line anchor-echo stripping to trigger empty-insert validation

- Fix trailing boundary-echo stripping for boundary-only payloads

- Extract shared chunking logic to hashline-chunk-formatter

- Align empty stream/iterable handling with formatHashLines

- Add regression tests for all fixes

src/tools/hashline-edit/hashline-chunk-formatter.ts

@@ -0,0 +1,52 @@
+export interface HashlineChunkFormatter {
+  push(formattedLine: string): string[]
+  flush(): string | undefined
+}
+
+interface HashlineChunkFormatterOptions {
+  maxChunkLines: number
+  maxChunkBytes: number
+}
+
+export function createHashlineChunkFormatter(options: HashlineChunkFormatterOptions): HashlineChunkFormatter {
+  const { maxChunkLines, maxChunkBytes } = options
+  let outputLines: string[] = []
+  let outputBytes = 0
+
+  const flush = (): string | undefined => {
+    if (outputLines.length === 0) return undefined
+    const chunk = outputLines.join("\n")
+    outputLines = []
+    outputBytes = 0
+    return chunk
+  }
+
+  const push = (formattedLine: string): string[] => {
+    const chunksToYield: string[] = []
+    const separatorBytes = outputLines.length === 0 ? 0 : 1
+    const lineBytes = Buffer.byteLength(formattedLine, "utf-8")
+
+    if (
+      outputLines.length > 0 &&
+      (outputLines.length >= maxChunkLines || outputBytes + separatorBytes + lineBytes > maxChunkBytes)
+    ) {
+      const flushed = flush()
+      if (flushed) chunksToYield.push(flushed)
+    }
+
+    outputLines.push(formattedLine)
+    outputBytes += (outputLines.length === 1 ? 0 : 1) + lineBytes
+
+    if (outputLines.length >= maxChunkLines || outputBytes >= maxChunkBytes) {
+      const flushed = flush()
+      if (flushed) chunksToYield.push(flushed)
+    }
+
+    return chunksToYield
+  }
+
+  return {
+    push,
+    flush,
+  }
+}