Add log-spaced checkpoint schedule

[amazloumi]

Summary

• Add CheckpointConfig.schedule ("interval" | "log", default "interval") and log_until (default 512). In log mode, should_save() saves at step 0 and each power of two up to log_until, then every interval step thereafter — the Pythia/PolyPythias cadence for studying early-training dynamics.
• Add CheckpointConfig.should_save(step) and route the save decision in scripts/train.py through it (was inline step % interval == 0). Default interval mode is byte-for-byte unchanged.
• Relax the keep_last_n guard to allow <= 0 = "retain all checkpoints". CheckpointManager._cleanup() already early-returns on keep_last_n <= 0; this exposes it via config so dense early checkpoints survive retention during a dynamics study.
• Add unit tests for both schedules, keep-all retention, and log_until validation.

Testing

• uv run ruff check kempnerforge/ tests/ passes
• uv run ruff format --check kempnerforge/ tests/ scripts/ passes
• uv run pyright kempnerforge/ passes (0 errors)
• uv run pytest tests/unit/ -v --timeout=60 passes
• If distributed code changed: uv run torchrun --nproc_per_node=4 -m pytest tests/distributed/ -v
• If training loop / parallelism / optimizers changed: uv run pytest tests/e2e/ --e2e -v

Closes #111

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
kempnerforge/config/checkpoint.py	100.00% <100.00%> (+8.69%)	⬆️

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[Naeemkh]

Please update the CHANGELOG.md file.

[Naeemkh]

There are three related parameter that is not clear to the user how to use them.

• In the schedule, without the comment, the user cannot understand if it is base 2 or base 10. May be change the log to log2.
• The log_until, gives the impression that it is just up to this point, but based on the comment it is only valid for the dynamic logging.
• If you use keep_last_n, regardless of the previous parameters, all logs will be deleted except the last 3 of them.

Please take a look at them.