fix(pack-session): advance oversized line skips

[ohdearquant]

Summary

• checkpoint bounded discarded prefixes while skipping oversized JSONL lines
• resume persisted mid-line skips after restart without parsing line fragments
• add multi-window forward-progress coverage and update the ingest contract guide

Testing

• cargo fmt -p khive-pack-session -- --check
• cargo clippy -p khive-pack-session --all-targets -- -D warnings
• cargo test -p khive-pack-session

Fixes #2010