🎨 Enhance RL feature vector

[flowerthrower]

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Description

Extends the reinforcement-learning observation with normalized frequencies for individual OpenQASM gates and measurements.

The gate schema remains local to the RL module. The upgrade guide notes that models trained with the previous observation space must be retrained.

Fixes #674

Stack

Depends on #757.

Validation

• uvx nox -s lint
• RL helper tests: 6 passed

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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