Address review: scripted registration block in README, old-layout import fallback in the checkpoint exporter

- README: steps 1-3 of the build are now also a copy-paste block (symlink
  + sed into STANDARD_PACKAGES + append include), verified end-to-end on a
  pristine stable_22Jul2025_update5 tree (configure + style_pair.h carries
  the pair style).
- export_matgl_checkpoint.py: try the matgl 4.0.3 layout first
  (matgl.ext.lammps), fall back to the pre-4.0.3 layout with the
  create_line_graph_torch shim -- the fallback branch is the code this PR
  previously replaced, kept verbatim.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: Longyun Shen

lammps/README.md

@@ -78,6 +78,19 @@ cmake -B build -S <lammps>/cmake \
 cmake --build build -j 8
 ```
 
+Steps 1–3 as a copy-paste block (GNU sed; the `set(STANDARD_PACKAGES`
+opener has kept this exact form across recent LAMMPS releases, and the
+list is not order-sensitive):
+
+```bash
+MATGL=/path/to/matgl
+LMP=/path/to/lammps
+ln -s "$MATGL/lammps/src/ML-MATGL" "$LMP/src/ML-MATGL"
+sed -i '/^set(STANDARD_PACKAGES$/a\  ML-MATGL' "$LMP/cmake/CMakeLists.txt"
+echo "include($MATGL/lammps/cmake/ML-MATGL.cmake)" >> "$LMP/cmake/CMakeLists.txt"
+grep -c ML-MATGL "$LMP/cmake/CMakeLists.txt"   # expect 2: package list + include
+```
+
 Check the registration before running anything:
 
 ```bash

lammps/src/ML-MATGL/export_matgl_checkpoint.py

@@ -20,9 +20,16 @@
 # (matgl.graph._compute_pyg, matgl.ext._lammps) predate that layout; the first
 # no longer exists, so the import raised ModuleNotFoundError and made the
 # "point pair_coeff at a checkpoint directory" path unusable.
-# _compute exports create_line_graph_torch directly,
-# so the in-memory shim that used to sit here is no longer needed either.
-from matgl.ext.lammps import export_lammps_model
+# The except branch keeps the script usable on the older layout, where
+# matgl.ext._lammps needs create_line_graph_torch shimmed in first.
+try:
+    from matgl.ext.lammps import export_lammps_model
+except ModuleNotFoundError:
+    import matgl.graph._compute_pyg as _cpyg
+
+    if not hasattr(_cpyg, "create_line_graph_torch"):
+        _cpyg.create_line_graph_torch = _cpyg.create_line_graph
+    from matgl.ext._lammps import export_lammps_model
 
 checkpoint_dir, out_path = sys.argv[1], sys.argv[2]
 potential = matgl.load_model(checkpoint_dir)