Skip ground classification when DTM already exists

If the DTM .tif exists and --force is not set, skip both ground
classification and DTM generation entirely. Previously, the pipeline
would spend 3+ minutes reclassifying ground even when the DTM was
already present and would be reused anyway.

Also includes: SharedDEM cache, enhanced WebP cartouche (compass rose,
adaptive scale bar, enriched info bar), removed COG/viewer, UTF-8
fix for parallel workers, skip logic for DTM and PDF.

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

lidar_pipeline/dtm.py

@@ -250,7 +250,7 @@ def classify_ground(laz_file, temp_dir, method='auto', force=False):
         return None
 
 
-def create_dtm_fast(las_file, basename, dtm_dir, resolution):
+def create_dtm_fast(las_file, basename, dtm_dir, resolution, force=False):
     """Create DTM using fast binning method with gap filling.
 
     Args:
@@ -258,10 +258,17 @@ def create_dtm_fast(las_file, basename, dtm_dir, resolution):
         basename: Base name for output file.
         dtm_dir: Directory for output DTM GeoTIFF.
         resolution: Grid resolution in meters per pixel.
+        force: If True, regenerate even if DTM already exists.
 
     Returns:
         Path to output DTM GeoTIFF, or None on failure.
     """
+    output_tif = dtm_dir / f"{basename}_dtm.tif"
+
+    if output_tif.exists() and not force:
+        logger.info(f"  DTM déjà existant — fichier réutilisé: {output_tif.name}")
+        return output_tif
+
     import laspy
 
     logger.info("  → Génération DTM...")