Playground: fix corrupted file sizes in ls output

browser_wasi_shim's Filestat.write_bytes() uses setUint8 for the
1-byte filetype field at offset 16, leaving bytes 17-23 unwritten.
Since Rust allocates the stat buffer with MaybeUninit, those padding
bytes contain stack garbage that corrupts the struct.

Fix by replacing write_bytes with a version that writes filetype as
a full 8-byte BigUint64 (zero-extended), clearing the padding.
Also add a test asserting file sizes are reasonable.

static/js/wasm-terminal.js

@@ -285,6 +285,27 @@ async function runCommand(argv, stdinData = "") {
     return 0;
   };
 
+  // Workaround: browser_wasi_shim Filestat.write_bytes() only writes the
+  // defined fields and skips padding bytes (e.g. bytes 17-23 between the
+  // 1-byte filetype and the 8-byte nlink). Rust allocates the stat buffer
+  // with MaybeUninit (uninitialized stack memory), so those padding bytes
+  // contain garbage that corrupts the struct when Rust reads it back.
+  // Fix: patch write_bytes to zero padding before writing fields.
+  // Monkey-patch write_bytes to also write the padding/nlink fields that
+  // the original skips. The original writes filetype as a single byte at
+  // ptr+16 but leaves bytes 17-23 untouched. On WASM those bytes come from
+  // uninitialized stack memory (MaybeUninit) and corrupt the struct.
+  wasiShim.wasi.Filestat.prototype.write_bytes = function(view, ptr) {
+    view.setBigUint64(ptr, this.dev, true);
+    view.setBigUint64(ptr + 8, this.ino, true);
+    view.setBigUint64(ptr + 16, BigInt(this.filetype), true); // zero-extends, clearing padding
+    view.setBigUint64(ptr + 24, this.nlink, true);
+    view.setBigUint64(ptr + 32, this.size, true);
+    view.setBigUint64(ptr + 40, this.atim, true);
+    view.setBigUint64(ptr + 48, this.mtim, true);
+    view.setBigUint64(ptr + 56, this.ctim, true);
+  };
+
   let exitCode = 0;
   try {
     const result = await WebAssembly.instantiate(wasmModule, {

static/js/wasm-terminal.test.html

@@ -453,6 +453,10 @@ async function runTests() {
       lsAl.includes(".."), true);
     assert("ls -al does not show 1970 date",
       lsAl.includes("1970"), false);
+    // File sizes should be reasonable (< 1000 bytes for sample files)
+    const sizeMatch = lsAl.match(/(\d+)\s+\w+\s+\d+.*csv\.txt/);
+    assert("csv.txt size is reasonable",
+      sizeMatch && parseInt(sizeMatch[1]) < 1000, true);
 
     // ===== UTF-8 / multibyte WASM tests =====
     section("UTF-8 multibyte WASM");