fix(repl): don't skip piped tests on Windows, fix ban-words

The piped stdin tests don't use Bun.Terminal and should work on
Windows. Only the interactive terminal tests need todoIf(isWindows).
Fix banned word: use .{0} ** 256 instead of = undefined for buffer.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/repl.zig

@@ -682,7 +682,7 @@ terminal_height: u16 = 24,
 ctrl_c_pressed: bool = false,
 
 // Buffered stdin
-stdin_buf: [256]u8 = undefined,
+stdin_buf: [256]u8 = .{0} ** 256,
 stdin_buf_start: usize = 0,
 stdin_buf_end: usize = 0,
 

test/js/bun/repl/repl.test.ts

@@ -134,7 +134,7 @@ async function withTerminalRepl(
   if (!proc.killed) proc.kill();
 }
 
-describe.todoIf(isWindows)("Bun REPL", () => {
+describe("Bun REPL", () => {
   describe("basic evaluation", () => {
     test("evaluates simple expression", async () => {
       const { stdout, exitCode } = await runRepl(["1 + 1", ".exit"]);