The cut margin, in model units: a set amount added to EVERY wire’s
routed length to give the cut length its BOM line carries (MF QTY) —
once per wire, never a percentage and never once per BOM line. It lives
here, in the typed block, because the block is rewritten from this struct
on every harness edit and a key it did not know would be dropped.
The router never reads it and [harness_fingerprint] deliberately does
not hash it: a margin change must not invalidate a route, so the tail
replays its cache and only the BOM’s arithmetic moves. Omitted from the
saved block at 0, so a document that never set one saves as before.