Add LAZ integrity check to skip corrupted files early

Validate file readability before PDAL classification. Corrupted/truncated
files are detected instantly via laspy header read and skipped with a clear
error message pointing to re-download, instead of wasting time on PDAL
and repair attempts that will fail anyway.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lidar_pipeline/dtm.py

@@ -124,6 +124,29 @@ def create_csf_pipeline(input_laz, output_las):
     return _create_ground_pipeline(input_laz, output_las, 'csf')
 
 
+def validate_laz(laz_file):
+    """Quick integrity check for a LAZ/LAS file.
+
+    Reads the header with laspy to detect truncated or corrupted files
+    before launching expensive PDAL processing.
+
+    Returns:
+        True if file is readable, False otherwise.
+    """
+    import laspy
+    try:
+        with laspy.open(str(laz_file)) as f:
+            # Just read the header — fast and catches truncated files
+            header = f.header
+            _ = header.point_count
+        return True
+    except Exception as e:
+        logger.error(f"  ✗ Fichier corrompu ou incomplet: {laz_file.name}")
+        logger.error(f"    {e}")
+        logger.error(f"    → Re-télécharger depuis https://ign.fr/lidar-hd")
+        return False
+
+
 def detect_ground_method(laz_file):
     """Detect the best ground classification method based on point cloud statistics.