fix: broaden separator regex to catch whitespace chars after -------

Change regex from /^-------\S/ to /^-------.+/ so that cases like
"-------\tapi.POST(" where whitespace follows the separator are also
detected as malformed. Add test for tab-prefixed content case.

src/core/diff/strategies/__tests__/multi-search-replace.spec.ts

@@ -155,6 +155,21 @@ describe("MultiSearchReplaceDiffStrategy", () => {
 			expect(result.error).toContain("some content here")
 		})
 
+		it("detects malformed separator with whitespace (tab) before content on the same line", () => {
+			const diff =
+				"<<<<<<< SEARCH\n" +
+				":start_line:70\n" +
+				"-------\tapi.POST(\n" +
+				"=======\n" +
+				"new content\n" +
+				">>>>>>> REPLACE"
+			const result = strategy["validateMarkerSequencing"](diff)
+			expect(result.success).toBe(false)
+			expect(result.error).toContain("separator line '-------'")
+			expect(result.error).toContain("must be on its own line")
+			expect(result.error).toContain("\tapi.POST(")
+		})
+
 		it("allows properly formatted separator on its own line", () => {
 			const diff =
 				"<<<<<<< SEARCH\n" +

src/core/diff/strategies/multi-search-replace.ts

@@ -217,7 +217,7 @@ export class MultiSearchReplaceDiffStrategy implements DiffStrategy {
 					if (marker === REPLACE) return reportInvalidDiffError(REPLACE, SEP)
 					if (marker.startsWith(REPLACE_PREFIX)) return reportMergeConflictError(marker, SEARCH)
 					// Detect malformed separator: `-------` with content on the same line (missing newline)
-					if (/^-------\S/.test(marker)) {
+					if (/^-------.+/.test(marker)) {
 						const trailingContent = marker.slice(7)
 						return {
 							success: false,