Fix dangerous pkill, circular import, and dedup _res_suffix

- Scope pkill to 'pdal pipeline' (avoids killing unrelated PDAL jobs)
- Inline _strip_lidar_ext in dtm.py to remove circular import from pipeline
- _res_suffix_str now delegates to LidarArchaeoPipeline._res_suffix

lidar_pipeline/cli.py

@@ -396,8 +396,10 @@ def main():
 def _kill_orphan_pdal(signum=None, frame=None):
     """Kill orphan PDAL processes on interrupt or exit."""
     import subprocess
+    # Belt-and-suspenders: os.killpg below is the primary mechanism.
+    # This handles the edge case where a child escapes the process group.
     try:
-        subprocess.run(["pkill", "-9", "-f", "pdal"], capture_output=True, timeout=3)
+        subprocess.run(["pkill", "-9", "-f", "pdal pipeline"], capture_output=True, timeout=3)
     except Exception:
         pass
     if signum is not None: