[Demirrr]

Third of the print to logger cleanup PRs (Improvements.md §3), after #656 and #657.

• learners/nero.py: verbose > 0 messages become logger.info, verbose > 1 ones logger.debug (gates kept). The "queue exceeded max, stopping search" notice is now an unconditional logger.warning, since it explains why the search ended early.
• nces_trainer.py, clip_trainer.py: progress/summary prints become logger.info; the decorative #/* banners and blank-line spacers are collapsed into single messages.
• Bug fix: both trainers had raise ValueError followed by print('Unsupported optimizer'). The print was unreachable, so the message never showed. It is now ValueError(f"Unsupported optimizer: {optimizer}").
• Improvements.md offender list refreshed from an AST count (202 to 155 real print calls; the old per-file numbers were stale); CHANGELOG updated.

Local: test_nero_simple, test_nces_trainer, test_clip_trainer, test_clip, test_nces, test_roces pass; ruff clean. Note: on a machine with a GPU, 6 test_nero_simple tests fail with a CPU/CUDA device mismatch, identically on unmodified develop (so unrelated to this PR); they pass with CUDA_VISIBLE_DEVICES="", which is what CI effectively has.

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