fix(hashline): prevented terminal newline from exposing editable blank line

- Prevented `formatNumberedLines` from appending a synthetic empty line when encountering a terminal newline.
- Added tests to ensure trailing newlines do not generate addressable blank lines.

packages/hashline/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Stopped `formatNumberedLines` from exposing a terminal newline as an editable blank line; deleting that synthetic row previously produced a misleading no-op.
+
 ### Added
 
 - Post-apply parse advisory: when the applied result stops parsing while the pre-edit content parsed, the response now carries a warning naming the first changed line. This catches balance-neutral misplacements — a statement swapped onto the wrong line number leaves no delimiter anomaly for the repair heuristics — which previously applied with zero feedback and surfaced only at the next compile.

packages/hashline/src/format.ts

@@ -139,8 +139,13 @@ export function formatNumberedLine(lineNumber: number, line: string): string {
 	return `${lineNumber}${HL_LINE_BODY_SEP}${line}`;
 }
 
-/** Format file text with hashline-mode line-number prefixes for display. */
+/**
+ * Format file text with hashline-mode line-number prefixes for display.
+ * A terminal newline terminates the preceding line; it is not an addressable
+ * blank line.
+ */
 export function formatNumberedLines(text: string, startLine = 1): string {
 	const lines = text.split("\n");
+	if (lines.length > 1 && lines[lines.length - 1] === "") lines.pop();
 	return lines.map((line, i) => formatNumberedLine(startLine + i, line)).join("\n");
 }

packages/hashline/test/format-v2.test.ts

@@ -1,5 +1,12 @@
 import { describe, expect, it } from "bun:test";
-import { applyEdits, parseLid, parsePatch, parsePatchStreaming, Tokenizer } from "@oh-my-pi/hashline";
+import {
+	applyEdits,
+	formatNumberedLines,
+	parseLid,
+	parsePatch,
+	parsePatchStreaming,
+	Tokenizer,
+} from "@oh-my-pi/hashline";
 
 function applyPatch(text: string, diff: string): string {
 	return applyEdits(text, parsePatch(diff).edits).text;
@@ -98,6 +105,11 @@ describe("hashline format v4", () => {
 		expect(applyEdits("a\nb\n", edits).text).toBe("a\nb\n");
 	});
 
+	it("does not expose the terminal newline as an editable blank line", () => {
+		expect(formatNumberedLines("a\nb\n")).toBe("1:a\n2:b");
+		expect(formatNumberedLines("a\nb\n\n")).toBe("1:a\n2:b\n3:");
+	});
+
 	it("treats a cut range ending at the trailing sentinel as ending at the last real line", () => {
 		const edits = parsePatch("CUT 2-3").edits;
 		expect(applyEdits("a\nb\n", edits).text).toBe("a\n");