Avoid overlapping MPP ancilla coordinates

[masa10-f]

Summary

• detect when an automatically inferred MPP ancilla centroid is already occupied
• keep the inferred temporal coordinate and deterministically search nearby XY positions for sufficient clearance
• preserve existing behavior for unoccupied centroids and explicitly supplied ancilla coordinates
• document the placement rule and add regression coverage for blocked fallback positions

Root cause

MPP ancillas without explicit coordinates were always assigned the component-wise centroid of their connected data nodes. The builder did not check whether another graph node already occupied that coordinate, so nodes could render on top of one another.

Impact

Coordinate-aware MPP imports now remain visually distinguishable when a support centroid coincides with a data qubit or another previously positioned node. Graph construction and temporal placement semantics are unchanged.

Validation

• python -m pytest -s -q — 797 passed, 1 skipped
• ruff format --check graphqomb/qec/qeccode.py tests/test_qec.py
• ruff check graphqomb/qec/qeccode.py tests/test_qec.py
• mypy graphqomb/qec/qeccode.py tests/test_qec.py
• pyright graphqomb/qec/qeccode.py tests/test_qec.py

[codecov]

Codecov Report

❌ Patch coverage is 80.76923% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.38%. Comparing base (455df96) to head (ba6ad20).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #238      +/-   ##
==========================================
- Coverage   86.41%   86.38%   -0.04%     
==========================================
  Files          31       31              
  Lines        5086     5111      +25     
  Branches      903      908       +5     
==========================================
+ Hits         4395     4415      +20     
- Misses        488      491       +3     
- Partials      203      205       +2     

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Check occupancy in the XY plane

When an inferred MPP ancilla shares the displayed data-plane position with an existing node but differs in z, this full 3D math.dist check treats the centroid as unoccupied and returns it unchanged. For example, a Type-I single-qubit Y support infers an ancilla at (x, y, 0.5) between data nodes at (x, y, 0) and (x, y, 1); visualize() projects 3D coordinates to XY, so all three nodes still render on top of each other despite the new placement rule. The occupancy and clearance tests should compare XY distance while preserving the inferred z coordinate.

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