feat: add amplitude damping noise model

[huhaha120]

Summary

• Add single-qubit and two-qubit amplitude damping Kraus channels.
• Add AmplitudeDampingNoise, TwoQubitAmplitudeDampingNoise, and AmplitudeDampingNoiseModel.
• Export the new noise model through graphix.noise_models.
• Add tests covering Kraus operators, noise-model transpilation, and density-matrix application.

Closes #497

Test Soundness

• tests/test_kraus.py checks that the one-qubit channel matches the Nielsen-Chuang amplitude damping Kraus operators and that the two-qubit channel is the tensor product of the one-qubit channel on both qubits.
• tests/test_noise_model.py checks that each pattern command receives the expected one- or two-qubit amplitude damping noise at the same insertion points as the existing depolarising model.
• tests/test_noisy_density_matrix.py checks physical action on density matrices:
  • an excited state decays to diag(gamma, 1-gamma);
  • a simple Hadamard MBQC pattern with measurement-channel damping matches the closed-form diagonal result.

Tests

uv run --python 3.11 --with-editable . --with pytest python -m pytest -o addopts='' tests/test_kraus.py tests/test_noise_model.py tests/test_noisy_density_matrix.py -q
uv run --python 3.11 --with-editable . --with ruff ruff check graphix/channels.py graphix/noise_models/amplitude_damping.py graphix/noise_models/__init__.py tests/test_kraus.py tests/test_noise_model.py tests/test_noisy_density_matrix.py
uv run --python 3.11 --with-editable . --with ruff ruff format --check graphix/channels.py graphix/noise_models/amplitude_damping.py graphix/noise_models/__init__.py tests/test_kraus.py tests/test_noise_model.py tests/test_noisy_density_matrix.py
git diff --check

Results:

• 49 passed
• All checks passed!
• 6 files already formatted
• git diff --check produced no output

Notes

• I used -o addopts='' locally because the repository's default pytest addopts include --benchmark-autosave, which requires the dev benchmark plugin. The focused tests above do not use benchmarking.
• AI assistance was used while drafting this change. I reviewed the implementation, corrected the analytical test expectation, and verified it with the tests and lint commands above.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.85%. Comparing base (6dd835a) to head (7fa44d5).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
graphix/noise_models/amplitude_damping.py	87.87%	8 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #522   +/-   ##
=======================================
  Coverage   88.85%   88.85%           
=======================================
  Files          49       50    +1     
  Lines        7135     7207   +72     
=======================================
+ Hits         6340     6404   +64     
- Misses        795      803    +8     

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