Log sweep cell substeps

[MaxGhenis]

Summary

• Add explicit per-cell progress logs for dense_sample, informed_l1, random_reweight, scoring, and checkpoint writes.
• Keep the matched-budget cell label consistent across start, substeps, and completion logs.
• Add a focused test covering the new substep log messages without running real optimization.

Tests

• uv run pytest tests/test_end_to_end.py
• uv run ruff check .
• uv run pytest