fix(datasets): backfill .static on caches predating it (PF test/predict)

[romeokienzler]

Problem

The PF test/predict path crashes on any dataset processed before .static was introduced:

File "gridfm_graphkit/tasks/pf_task.py", line 132, in test_step
    RemovePFMask()(batch)
File "gridfm_graphkit/datasets/masking.py", line 226, in forward
    ].static
AttributeError: 'NodeStorage' object has no attribute 'static'

RemovePFMask (in test_step / predict_step) restores static limit columns from the .static
snapshot that build_hetero_data attaches at graph-build time. But .static was added after many
datasets were already processed to disk. HeteroGridDatasetDisk.process() skips regeneration when a
processed_raw_files.done marker exists, so those stale caches are never rebuilt — their graphs carry
the limit columns only inside bus.x / branch edge_attr, and loading one crashes the PF
test/predict path.

This was surfaced by running the training integration tests (integrationtests/test_base_set.py::test_train_pf)
on main: the committed case14 fixture ships pre-processed data_index_*.pt graphs built before
.static existed, so every PF run aborts in the test phase. (OPF is unaffected — it doesn't use .static.)

Fix

• Add backfill_static(data) in datasets/graph_builder.py: if a graph lacks .static, reconstruct it
  from the raw limit columns still present in bus.x / branch edge_attr — identical to what
  build_hetero_data stores. Freshly-processed graphs already carry .static, so it's a no-op for them.
• Invoke it in HeteroGridDatasetDisk.get() right after HeteroData.from_dict and before
  normalisation, so the captured snapshot matches build-time raw semantics (and branch masking slices
  it consistently afterward).
• Centralise the limit-column indices as BUS_STATIC_COLS / BRANCH_STATIC_COLS so the build path and
  the backfill path share one source of truth.

No dataset regeneration required — stale caches become forward-compatible on load.

Validation

On vela (torch 2.13.0+cu130, H100), pytest integrationtests against the stale case14 fixture:
2 passed in 0:45:40 (PF + OPF), no AttributeError. Before the fix, PF crashed in the test phase.

🤖 Generated with Claude Code

[romeokienzler]

@romeokienzler thanks for the fix — the diagnosis is clear and the BUS_STATIC_COLS/BRANCH_STATIC_COLS single-source-of-truth refactor is a nice touch. The placement of backfill_static in get() (after from_dict, before normalisation) matches the build-time raw semantics.

What's needed

• Unit test for backfill_static. The integration run on vela is great validation, but per the contribution checklist new functionality needs a unit test in CI too. tests/test_remove_pf_mask.py already builds a graph with .static; a small companion test could (a) delete .static from a built graph and assert backfill_static reconstructs it to equal the original snapshot, and (b) assert it's a no-op when .static is already present. That locks in the exact behaviour you verified.
• CI still pending — pytests / pre-commit / CodeQL haven't reported yet; worth a glance once they land. DCO is signed off ✅. Note the repo-wide pip-audit failure (PYSEC-2026-3624 / lightning) is a known infra blocker being fixed separately — not this PR's fault.

Otherwise this looks tightly scoped and forward-compatible with no reprocessing required, which is the right call. Deferring the merge decision to a maintainer.

— 🤖 _automated pre-review; a maintainer will follow up_