Fix luni block: missing 4-byte character count field

The unicode layer name block must be [char_count(4B)][utf-16-be chars].
We wrote raw utf-16 without the count, so Photoshop read the first
characters as the length, shifted the entire extra-data parse, and every
layer's pixels came out empty (all layers transparent). Fixed: name now
reads back correctly and 167/167 layers match the rendered originals
(mean diff 0.000).

ppl2psd.py

@@ -508,9 +508,12 @@ def _layer_record(top, left, bottom, right, name, opacity, visible, channels,
     pad = (-len(extra)) % 4
     if pad:
         extra += b"\x00" * pad
-    # luni: unicode layer name
+    # luni: unicode layer name. Data = 4-byte CHARACTER count + utf-16-be
+    # chars. (Writing raw utf-16 without the count shifts the extra-data
+    # parse and Photoshop reads every layer's pixels as empty.)
     uni = name.encode("utf-16-be")
-    extra += b"8BIMluni" + struct.pack(">I", len(uni)) + uni
+    luni_data = struct.pack(">I", len(uni) // 2) + uni
+    extra += b"8BIMluni" + struct.pack(">I", len(luni_data)) + luni_data
     # standard blocks Photoshop expects (see _standard_blocks)
     extra += _standard_blocks(layer_id)
     rec += struct.pack(">I", len(extra))