Idea: fastCN

Given a node count `-n`, submit an interactive job of size `-n` for the longest amount of walltime available in backfill.

Given a time constraint `-W`, submit an interactive job for the largest size job available in backfill that can run for `-W`.

Start by reviewing the repository's support scripts and identifying the entry point for interactive job submission. Implement the `-n` behavior to request the longest available backfill walltime and the `-W` behavior to select the largest job that can run for the requested time; done means both command-line cases submit the intended job.

Written by the indexing model from the issue text.