Fix PSD structure: add missing LayerInfo length field and correct channel order

- Layer info section was missing its 4-byte length field, causing all
  subsequent offsets to shift and Photoshop to fail with 'unexpected end
  of file'
- Layer channel data was ordered [R,G,B,A] with ids [-1,0,1,2], shifting
  every channel by one (alpha held red, red held green, ...) - fixed to
  [A,R,G,B] so id -1 is the alpha mask
- Verified: psd_tools opens the PSD, per-layer pixels match the rendered
  originals with 0.000 mean difference

ppl2psd.py

@@ -432,9 +432,17 @@ def channel_blocks(img: Image.Image, workers: int,
             # zero RGB where alpha==0 for better compression
             arr = arr.copy()
             arr[arr[..., 3] == 0] = 0
-        planes = [arr[..., i].tobytes() for i in range(nch)]
+            # PSD layer channel ids: -1=alpha mask, 0=R, 1=G, 2=B
+            planes = [arr[..., 3].tobytes()] + [arr[..., i].tobytes() for i in range(3)]
+        else:
+            planes = [arr[..., i].tobytes() for i in range(nch)]
     else:
-        planes = [img.getchannel(c).tobytes() for c in img.getbands()]
+        bands = img.getbands()
+        if bands == ("R", "G", "B", "A"):
+            planes = [img.getchannel("A").tobytes()] + \
+                     [img.getchannel(c).tobytes() for c in "RGB"]
+        else:
+            planes = [img.getchannel(c).tobytes() for c in bands]
     w = img.width
     jobs = [(plane, w) for plane in planes]
     if workers > 1 and len(jobs) > 1:
@@ -518,10 +526,12 @@ def build_psd(layers, width: int, height: int, out_path: str, workers: int,
     fh.write(struct.pack(">I", 0))  # color mode data
     fh.write(struct.pack(">I", 0))  # image resources
 
-    layer_info = bytes(records) + bytes(payload)
-    count_payload = struct.pack(">h", len(layers)) + layer_info
-    lam = struct.pack(">I", len(count_payload) + 4) + count_payload
-    lam += struct.pack(">I", 0)  # global layer mask info
+    # Layer info section: [Length(4)] + [layer count(2) + records + channel data]
+    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)
+    lam = layer_info + struct.pack(">I", 0)  # global layer mask info
     fh.write(struct.pack(">I", len(lam)) + lam)
 
     # merged image data (RGB, RLE)