Fixes two bugs in delivery-note processing:
1. 1000x unit conversion error: slips quoted in kg but PO lines priced per ton.
2. Off-by-one that paired delivery lines to the wrong PO items.
All 8 plan tasks complete: units module, dn-item-row builder, route rewire,
dual-unit UI rendering, backfill migration (56/57 rows patched), and operational
cleanup. Final state: PO MEGCPO9165 line bxqbo8rs04pjqwc reconciles to
133.5 ton / R22,494.75 across 4 notes (not 1 515% or R10.9 m).
See docs/superpowers/plans/2026-09-17-delivery-note-unit-correctness.md.