Clarify improving explanations for flat trajectories

[Copilot]

The “Look for improvements” issue required better handling of cases where a metric is flat so explanations don’t mislabel it as increasing.

• Runtime logic: explain_improving now reports unchanged trajectories explicitly instead of treating them as increasing.
• Tests: Added coverage for flat trajectories to verify the updated explanation output.

Example:

# Previously a flat series could be described as increasing
explain_improving([1, 1, 1])  # now reports it as unchanged

Original prompt

Look for improvents

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