Fix blending ranges: zero composite range made PS skip layer blending

PS reads layer blending ranges to decide if a layer participates in
compositing. psd_tools wrote all-zero ranges (composite 0..0), which tells
Photoshop the layer has an empty blend range -> the canvas renders empty
even though layer pixels are correct (Ctrl+C/V shows content, export PNG is
transparent, save-as by PS keeps the bad ranges). Now writes full-range
(0..65535) composite + channel ranges like PS-generated files, verified
against the reference PSD.

ppl2psd.py

@@ -501,7 +501,10 @@ def _layer_record(top, left, bottom, right, name, opacity, visible, channels,
     # extra data
     extra = bytearray()
     extra += struct.pack(">I", 0)        # layer mask data length
-    extra += struct.pack(">I", 8) + b"\x00" * 8  # blending ranges (default)
+    # blending ranges: composite + 4 channels, ALL full range (0..65535).
+    # PS writes these as 0x0000/0xffff; a zero composite range tells PS the
+    # layer does not participate in blending -> the canvas renders empty.
+    extra += struct.pack(">I", 40) + b"\x00\x00\xff\xff\x00\x00\xff\xff" * 5
     nb = name.encode("utf-8")
     extra += bytes([min(len(nb), 255)]) + nb[:255]
     # pad pascal string to a multiple of 4 bytes
@@ -548,27 +551,30 @@ def build_psd(layers, width: int, height: int, out_path: str, workers: int,
     for name, opacity, visible, (lx, ly), img in layers:
         if visible and opacity > 0:
             merged.paste(img, (lx, ly), img)
-    merged_rgb = merged.convert("RGB")  # transparent -> black
-    # 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).
+    merged_rgb = None  # keep the RGBA merged image for the 4-channel output
+    # Merged image data is written RAW with a single compression field and
+    # all planes in planar order (R G B A). 4 channels because the document
+    # has transparency (negative layer count references the merged alpha).
     if np is not None:
-        arr = np.asarray(merged_rgb)
+        arr = np.asarray(merged)
         merged_block = b"\x00\x00" + b"".join(
-            arr[..., i].tobytes() for i in range(3))
+            arr[..., i].tobytes() for i in range(4))
     else:
         merged_block = b"\x00\x00" + b"".join(
-            merged_rgb.getchannel(c).tobytes() for c in "RGB")
+            merged.getchannel(c).tobytes() for c in "RGBA")
 
-    # assemble file
+    # assemble file. Header channels = 4 (document has transparency, like PS
+    # writes when layers have alpha).
     fh = open(out_path, "wb")
-    fh.write(struct.pack(">4sH6sHIIHH", b"8BPS", 1, b"\x00" * 6, 3, height, width, 8, 3))
+    fh.write(struct.pack(">4sH6sHIIHH", b"8BPS", 1, b"\x00" * 6, 4, height, width, 8, 3))
     fh.write(struct.pack(">I", 0))  # color mode data
     fh.write(struct.pack(">I", 0))  # image resources
 
     # Layer info section: [Length(4)] + [layer count(2) + records + channel data]
-    layer_info_body = struct.pack(">h", len(layers)) + bytes(records) + bytes(payload)
+    # Negative layer count: abs = layer count, first alpha channel of the
+    # merged image holds the merged transparency (PS writes this when the
+    # document has transparent layers).
+    layer_info_body = struct.pack(">h", -len(layers)) + bytes(records) + bytes(payload)
     layer_info = struct.pack(">I", len(layer_info_body)) + layer_info_body
     # layer & mask section: length = layer info + global layer mask info
     # (the outer length does NOT include its own 4-byte length field)