Add non-unitary parity projection example

[masa10-f]

Summary

• Add examples/nonunitary_parity_projection.py: a 3-node star graph demonstrating the Kraus branch $K_s = (I + (-1)^s Z_0 Z_1)/2$ via ancilla X-measurement
• Starting from $|{+}{+}\rangle$, the two measurement branches produce Bell states ($|\Phi^+\rangle$ for $s=0$, $|\Psi^+\rangle$ for $s=1$), showing measurement-induced entanglement as a non-unitary MBQC operation
• Uses GraphState.from_graph() + qompile() + PatternSimulator (StateVector), with graph visualization
• Update CHANGELOG.md

Test plan

• python examples/nonunitary_parity_projection.py runs without error and produces correct overlap outputs
• pytest passes (no regressions)
• ruff check / ruff format --check pass
• make html in docs/ builds without warnings

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.60%. Comparing base (eaa6f69) to head (f23a559).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #182      +/-   ##
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+ Coverage   84.54%   84.60%   +0.06%     
==========================================
  Files          24       24              
  Lines        3203     3203              
  Branches      550      550              
==========================================
+ Hits         2708     2710       +2     
+ Misses        362      361       -1     
+ Partials      133      132       -1     

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