fix(hashline): recovered dangling range separators in hunk headers

- Recover dangling range separators in hunk headers as single-line ranges instead of rejecting them.
- Ensure strict rejection is maintained when a dangling separator is followed by invalid tokens.

packages/hashline/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Recovered dangling range separators in hunk headers (`PUT 244.=:`, `CUT 5.=`) as single-line ranges (`N.=N`) instead of rejecting the header as an orphan payload line.
+
 ## [17.3.0] - 2026-08-13
 
 ### Fixed

packages/hashline/src/tokenizer.ts

@@ -182,6 +182,37 @@ function scanRangeSeparator(line: string, index: number, end: number): number |
 	return cursor;
 }
 
+/**
+ * Recover a dangling range separator: the run after `N` contains at least one
+ * non-whitespace separator char but no end number (`244.=:`, `5-`, `12.. @reg`).
+ * Models write this intending an open range; it collapses to `N.=N`. Returns
+ * the index past the run only when what follows is `:`, `@`, or end-of-header —
+ * anything else keeps the header on the strict rejection path.
+ */
+function scanDanglingSeparator(line: string, index: number, end: number): number | null {
+	let cursor = index;
+	let sawSeparatorChar = false;
+	while (cursor < end) {
+		const code = line.charCodeAt(cursor);
+		if (code === CHAR_HYPHEN || code === CHAR_DOT || code === CHAR_EQUALS || code === CHAR_ELLIPSIS) {
+			sawSeparatorChar = true;
+			cursor++;
+			continue;
+		}
+		if (isWhitespaceCode(code)) {
+			cursor++;
+			continue;
+		}
+		break;
+	}
+	if (!sawSeparatorChar) return null;
+	if (cursor < end) {
+		const code = line.charCodeAt(cursor);
+		if (code !== CHAR_COLON && code !== CHAR_AT) return null;
+	}
+	return cursor;
+}
+
 function scanHeaderRange(line: string, index = 0, end = trimEndIndex(line), allowSingle = false): RangeScan | null {
 	const numberStart = skipWhitespace(line, index, end);
 	const start = scanLineNumber(line, numberStart, end);
@@ -189,6 +220,14 @@ function scanHeaderRange(line: string, index = 0, end = trimEndIndex(line), allo
 	const afterFirst = scanRangeSeparator(line, start.nextIndex, end);
 	if (afterFirst === null) {
 		if (!allowSingle) return null;
+		const dangling = scanDanglingSeparator(line, start.nextIndex, end);
+		if (dangling !== null) {
+			return {
+				range: { start: { line: start.line }, end: { line: start.line } },
+				nextIndex: dangling,
+				hadSeparator: true,
+			};
+		}
 		return {
 			range: { start: { line: start.line }, end: { line: start.line } },
 			nextIndex: skipWhitespace(line, start.nextIndex, end),

packages/hashline/test/leniency.test.ts

@@ -82,6 +82,16 @@ describe("hashline core — verb header forms", () => {
 		expect(applyPatch(FILE, "CUT 2")).toBe("a\nc\nd\ne");
 	});
 
+	it("recovers a dangling range separator as a single-line range", () => {
+		expect(applyPatch(FILE, "PUT 2.=:\n+X")).toBe("a\nX\nc\nd\ne");
+		expect(applyPatch(FILE, "PUT 2-:\n+X")).toBe("a\nX\nc\nd\ne");
+		expect(applyPatch(FILE, "CUT 2.=")).toBe("a\nc\nd\ne");
+	});
+
+	it("still rejects a dangling separator followed by junk", () => {
+		expect(() => parsePatch("PUT 2.= junk:\n+X")).toThrow();
+	});
+
 	it("recovers top-level numbered snapshot rows as single-line replacements", () => {
 		for (const separator of [":", "|"]) {
 			const result = parsePatch(`2${separator}B\n4${separator}D`);