🔥 Remove CircuitOptimizer

[denialhaag]

Description

🤖 AI text below 🤖

Backports the removal of CircuitOptimizer to v3.x:

• ♻️ Remove CircuitOptimizer #2262

Circuit flattening and final-measurement removal move to QuantumComputation. Equivalence-checking transformations move to MQT QCEC, mapping transformations move to MQT QMAP, and both downstream packages provide the single-qubit gate fusion they require. The unused public circuit dependency graph and transformations without production consumers are removed.

AI notice

This PR and its contents were created with the assistance of GPT-5.6 Sol via Codex.

Checklist

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• I have added entries to the changelog for any noteworthy additions, changes, fixes, or removals.
• I have added migration instructions to the upgrade guide (if needed).
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

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[codecov]

Codecov Report

❌ Patch coverage is 94.49541% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/ir/CircuitOptimization.cpp	94.4%	6 Missing ⚠️

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