refactor: Refactor MACE wrapper helper code

[njzjz]

Summary

• move MACE network construction, cuEquivariance setup, freeze-time conversion, and state-linking helpers into deepmd_gnn.mace_network
• share export, DeePMD op placeholder, and observed-type stat compatibility helpers across MACE and NequIP wrappers
• add development notes documenting wrapper module boundaries and the MACE lower-forward stages

Tests

• ruff check deepmd_gnn docs
• python -m compileall -q deepmd_gnn
• pytest -q tests/test_mace_comm.py tests/test_pt_expt.py
• pytest -q tests/test_nequip_comm.py tests/test_mace_off.py -m 'not slow'\n- pytest -q tests/test_model.py::TestMaceModel\n- commit hook: mypy, ruff, ruff format, whitespace checks\n

[codecov]

Codecov Report

❌ Patch coverage is 55.82822% with 72 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.98%. Comparing base (9399f06) to head (6bb02b8).

Files with missing lines	Patch %	Lines
deepmd_gnn/mace_network.py	38.75%	49 Missing ⚠️
deepmd_gnn/stat_compat.py	56.52%	10 Missing ⚠️
deepmd_gnn/deepmd_ops.py	50.00%	4 Missing ⚠️
deepmd_gnn/export.py	90.90%	3 Missing ⚠️
deepmd_gnn/mace.py	75.00%	3 Missing ⚠️
deepmd_gnn/nequip.py	57.14%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #136      +/-   ##
==========================================
+ Coverage   78.52%   78.98%   +0.45%     
==========================================
  Files          12       16       +4     
  Lines        1574     1589      +15     
==========================================
+ Hits         1236     1255      +19     
+ Misses        338      334       -4     

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