fix: preserve legacy text across trans helpers

scripts/src/apply-patch-v2.test.ts

@@ -167,6 +167,50 @@ describe("apply-patch v2 fs bulk update path", () => {
     }
   });
 
+  test("preserves legacy single-byte content during ASCII localized replacement", async () => {
+    const original = Buffer.concat([
+      Buffer.from("static int old_value = 1;\r\n// legacy ", "ascii"),
+      Buffer.from([0xca]),
+      Buffer.from(" comment\r\n", "ascii"),
+    ]);
+    let captured: ApplyPatchV2BulkReplacementWritePlan | undefined;
+    const fs: ApplyPatchV2FileSystem = {
+      async stat() { throw new Error("stat should not be used"); },
+      async readBlock() { throw new Error("readBlock should not be used"); },
+      async writeFile() { throw new Error("writeFile should not be used"); },
+      async deleteFile() { throw new Error("deleteFile should not be used"); },
+      async readFiles(paths) {
+        return new Map(paths.map((path) => [path, {
+          content: original.toString("latin1"),
+          bytes: original,
+          encodingStyle: "legacy-single-byte" as const,
+        }]));
+      },
+      async applyReplacementsBulk(_paths, plans) {
+        captured = plans.get("src/legacy.c");
+      },
+    };
+    const exitCode = await runApplyPatchV2({
+      executor: { fs },
+      stdin: Readable.from([[
+        "*** Begin Patch",
+        "*** Update File: src/legacy.c",
+        "@@",
+        "-static int old_value = 1;",
+        "+static int new_value = 2;",
+        "*** End Patch",
+      ].join("\n")]),
+      stdout: new CaptureWritable(),
+      stderr: new CaptureWritable(),
+    });
+
+    expect(exitCode).toBe(0);
+    expect(captured?.encodingStyle).toBe("legacy-single-byte");
+    expect(captured?.newlineStyle).toBe("crlf");
+    expect(captured?.originalBytes).toBe(original.length);
+    expect(captured?.originalSha256).toBe(createHash("sha256").update(original).digest("hex"));
+  });
+
   test("bulk read failure falls back to block reads but still writes through replacement bulk", async () => {
     const files = new Map<string, string>([
       ["a.md", "first\nold-a\nlast\n"],