Add Stim circuit importer

[masa10-f]

Summary

• Add a Stim importer that converts supported noiseless Clifford gate blocks and tick-separated MPP blocks into GraphQOMB patterns.
• Preserve sparse Stim qubit mappings and QUBIT_COORDS metadata in the import result.
• Add an opt-in QEC graph builder mode that exposes data layers as graph inputs/outputs for fragment composition.

Validation

• uv run --extra stim pytest tests/test_stim_importer.py tests/test_stim_mpp.py tests/test_qec.py -q
• uv run --extra stim pytest tests/test_stim_compiler.py tests/test_ptn_format.py::test_roundtrip_preserves_logical_observables_for_stim tests/test_ptn_format.py::test_roundtrip_preserves_detectors_for_stim -q
• uv run --no-sync pyright graphqomb/stim_importer.py
• uv run --no-sync ruff check

[codecov]

Codecov Report

❌ Patch coverage is 37.28814% with 148 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (feat/qec-helper@be48e39). Learn more about missing BASE report.

❌ Your patch check has failed because the patch coverage (37.28%) is below the target coverage (75.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Avoid creating identity wires for every tick block

When a tick-separated unitary block touches only some of the global Stim qubits, such as H 10 before a later MPP X10*Z12, this full-width Circuit creates an idle wire whose single graph node is both an input and an output. On the next compose for that same qindex, the matched output node is excluded and then remapped again as a graph1 input, leaving no node_map1 entry and causing the importer to raise KeyError instead of producing a pattern. Build each fragment only over the qubits present in that block, or handle these input/output identity nodes during composition.

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