Document that valid mask is all-True (dense game z-buffer)

README.md

@@ -53,7 +53,9 @@ Each example is one RGB frame paired with dense depth from the engine z-buffer:
 - `image` : RGB frame (1080x1920), PNG.
 - `depth_nearness` : relative log-nearness stored as a 16-bit PNG. Decode with
   `nearness = numpy.array(x) / 65535.0` (in [0,1], near = 1). This is relative, not metric.
-- `valid` : validity mask (0 or 255).
+- `valid` : per-pixel validity mask (0 or 255). Game z-buffers are fully dense, so for this dataset
+  every pixel is valid and the mask is all-255 (appears all-white in the Viewer). It is included only
+  for format compatibility with real-sensor depth datasets, which have holes; you can ignore it here.
 - `game`, `session`, `frame`, `split` : metadata.
 
 Splits (48,615 frames total):