Fix merged image data: single compression field + RAW for compatibility

Per Adobe PSD spec, the Image Data Section has ONE 2-byte compression
field followed by all planes in planar order (RRR GGG BBB). The previous
code wrote a compression marker per channel (4 extra bytes) and used RLE,
which third-party viewers parse incorrectly (stretching) and Photoshop
rejected. Now writes RAW data with a single compression marker, verified
by pytoshop (decompresses to 3x4882x9764).

ppl2psd.py

@@ -518,7 +518,17 @@ def build_psd(layers, width: int, height: int, out_path: str, workers: int,
         if visible and opacity > 0:
             merged.paste(img, (lx, ly), img)
     merged_rgb = merged.convert("RGB")  # transparent -> black
-    merged_channels = channel_blocks(merged_rgb, workers, progress)[0:3]
+    # Merged image data is written RAW (compression=0) for maximum
+    # compatibility with third-party viewers, which often only parse RAW
+    # merged data and stretch RLE data. Per spec there is ONE 2-byte
+    # compression field followed by all planes in planar order (RRR GGG BBB).
+    if np is not None:
+        arr = np.asarray(merged_rgb)
+        merged_block = b"\x00\x00" + b"".join(
+            arr[..., i].tobytes() for i in range(3))
+    else:
+        merged_block = b"\x00\x00" + b"".join(
+            merged_rgb.getchannel(c).tobytes() for c in "RGB")
 
     # assemble file
     fh = open(out_path, "wb")
@@ -534,8 +544,7 @@ def build_psd(layers, width: int, height: int, out_path: str, workers: int,
     lam = layer_info + struct.pack(">I", 0)  # global layer mask info
     fh.write(struct.pack(">I", len(lam)) + lam)
 
-    # merged image data (RGB, RLE)
-    merged_block = b"".join(p for _, p in merged_channels)
+    # merged image data (RGB, RAW)
     fh.write(merged_block)
     fh.close()