Add online CC cubing experiment workflow

[isPANN]

Summary

• add the structured Region cuber as a standalone Cube-and-Conquer frontend
• add the classical online CC difficulty cutoff D^2(D+I)/N
• calibrate Region thresholds by frontier size only
• compare Region and upstream march_cu frontiers with one auditable parallel conquer harness
• document the primary baseline and keep machine-specific Slurm wrappers out of the repository

Residual-CNF compilation and representation experiments are intentionally excluded from this PR.

Validation

• cargo test (103 tests passed)
• python3 -m unittest discover -s tests (28 tests passed)
• cargo fmt --check
• git diff --check