⚡ Use compact scalar RL observations

[flowerthrower]

🤖 AI text below 🤖

Description

Moves the compact scalar portion of #758 into its own stacked PR.

The num_qubits and depth RL observations become one-element float32 arrays in [0, 1]. Qubit count is scaled by the target device's qubit count; depth is log1p-scaled and capped at 999,999. Existing RL models must be retrained, and direct PredictorEnv observation consumers must handle the array values.

This PR is stacked on #758 and contains only the compact scalar encoding, its existing focused tests, and the corresponding changelog and upgrade-guide text.

Validated with uvx nox -s lint and the focused helper, environment-reset, and tracer tests (3 passed).

Related to #674

Part of #664

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 relevant local checks.
• I have reviewed my own code changes.

If PR contains AI-assisted content:

• Any agent that created, edited, or submitted GitHub content was explicitly authorized for that scope, as required by our AI Usage Guidelines.
• Every agent-authored or agent-edited public text body begins with the visible disclosure 🤖 *AI text below* 🤖 (titles are exempt).
• I have disclosed AI assistance in the PR description.
• I confirm that I have personally reviewed and understood all AI-generated content, and accept full responsibility for it.

[flowerthrower]

🤖 AI text below 🤖\n\nFolded back into the already-reviewed #758 observation PR.

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/mqt/predictor/rl/predictorenv.py	66.6%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!