Fix shell cat panic on Windows when reader chunks arrive late

Fixes a race condition where IO reader chunks could arrive after the cat
builtin transitioned to terminal states (.done, .waiting_write_err, .idle),
causing a panic with "Invalid state". This was particularly reproducible on
Windows in package scripts.

The fix handles these terminal states gracefully in onIOReaderChunk() by
ignoring late-arriving chunks since the reader is already being cleaned up.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/shell/builtin/cat.zig

@@ -196,9 +196,10 @@ pub fn onIOReaderChunk(this: *Cat, chunk: []const u8, remove: *bool) Yield {
             _ = this.bltn().writeNoIO(.stdout, chunk);
             return .done;
         },
-        else => @panic("Invalid state"),
+        // Race condition: chunks can arrive after state transitions (especially on Windows)
+        // Just ignore them - the reader is being cleaned up
+        .done, .waiting_write_err, .idle => return .done,
     }
-    return .done;
 }
 
 pub fn onIOReaderDone(this: *Cat, err: ?jsc.SystemError) Yield {

test/regression/issue/23370.test.ts

@@ -0,0 +1,54 @@
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe, tempDirWithFiles } from "harness";
+
+test("cat should not panic on delayed reader chunks (issue #23370)", async () => {
+  const dir = tempDirWithFiles("cat-race", {
+    "file1.txt": "a".repeat(1024 * 1024), // 1MB
+    "file2.txt": "b".repeat(1024 * 1024), // 1MB
+    "file3.txt": "c".repeat(1024 * 1024), // 1MB
+    "test.js": `
+      const { $ } = require("bun");
+      await $\`cat file1.txt file2.txt file3.txt\`;
+    `,
+  });
+
+  const proc = Bun.spawn({
+    cmd: [bunExe(), "test.js"],
+    cwd: dir,
+    env: bunEnv,
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [exitCode, stderr] = await Promise.all([proc.exited, proc.stderr.text()]);
+
+  expect(stderr).not.toContain("panic");
+  expect(stderr).not.toContain("Invalid state");
+  expect(exitCode).toBe(0);
+}, 10000);
+
+test("cat in package script should not panic", async () => {
+  const dir = tempDirWithFiles("cat-pkgscript", {
+    "file.txt": "x".repeat(1024 * 1024), // 1MB
+    "package.json": JSON.stringify({
+      name: "test",
+      scripts: {
+        test: "cat file.txt",
+      },
+    }),
+  });
+
+  const proc = Bun.spawn({
+    cmd: [bunExe(), "run", "test"],
+    cwd: dir,
+    env: bunEnv,
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [exitCode, stderr] = await Promise.all([proc.exited, proc.stderr.text()]);
+
+  expect(stderr).not.toContain("panic");
+  expect(stderr).not.toContain("Invalid state");
+  expect(exitCode).toBe(0);
+}, 10000);