Feature Enhancement: Auto-detect & resume iteration from checkpoint files

[dotvignesh]

Implements feature enhancement request from #535:

Keep numbered adapter checkpoints monotonic when resuming with --resume-adapter-file.

Behavior (no new flags):

1. If the resume filename matches NNNNNN_adapters.safetensors, parse NNNNNN and continue from there (next save = NNNNNN + save_every).
2. Else, scan the target directory for the highest *_adapters.safetensors and continue from that number.
3. Else, start from 0. Unnumbered adapters/adapters.safetensors behavior is unchanged.