Implement amplitude damping noise model (closes #497)

[Vinny010]

Closes #497.

Adds amplitude damping noise to the density-matrix simulation stack.

Channels (graphix.channels)

• amplitude_damping_channel(gamma) and two_qubit_amplitude_damping_channel(gamma), returning the corresponding KrausChannel. The single-qubit channel uses the standard Kraus operators K₁ = diag(1, √(1−γ)), K₂ = [[0, √γ], [0, 0]]; the two-qubit channel is the tensor product of two independent single-qubit channels.

Noise model (graphix.noise_models.amplitude_damping)

• AmplitudeDampingNoise / TwoQubitAmplitudeDampingNoise (deriving from Noise) and AmplitudeDampingNoiseModel (deriving from NoiseModel), mirroring DepolarisingNoiseModel. Channel parameters use a _gamma suffix to reflect that they are damping rates rather than Pauli probabilities. All three are exported from graphix.noise_models.

Soundness of the tests

• Channel action: amplitude damping has the closed form Φ_γ(|0⟩⟨0|) = |0⟩⟨0| (ground state unaffected) and Φ_γ(|1⟩⟨1|) = γ|0⟩⟨0| + (1−γ)|1⟩⟨1| (excited state decays with probability γ); the action on |+⟩⟨+| and the trace-preservation are checked against the analytic result. The two-qubit channel is verified to equal the Kronecker product of single-qubit actions on a product state.
• Per-step application: the model is checked to insert the expected ApplyNoise commands (carrying amplitude damping with the right gamma, nodes, and domain) after N/E/X/Z and before M, and for input nodes.
• End-to-end: a zero-damping model reproduces the noiseless result, and a single-preparation pattern with prepare_error_gamma reproduces the analytic Φ_γ(|+⟩⟨+|).

Passes ruff, mypy --strict, pyright, and pytest locally (new tests plus the existing noise/density-matrix suites).

Developed with LLM assistance, reviewed and tested by me.

[codecov]

Codecov Report

❌ Patch coverage is 91.89189% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.91%. Comparing base (fcdb8f4) to head (7885583).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
graphix/noise_models/amplitude_damping.py	90.90%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #525      +/-   ##
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+ Coverage   88.85%   88.91%   +0.05%     
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  Files          49       50       +1     
  Lines        7135     7209      +74     
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+ Hits         6340     6410      +70     
- Misses        795      799       +4     

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