How far outside its edges a block reaches into the lattice. The moat is
seeded a lane at a time at distance + 2 cells out, so the outermost lane —
and with it the last channel this block trims or creates — sits here.
The 1-cell clearance that pins occupy just outside a block edge; wires route
beyond it (the moat/channels start a further cell out). A straight wire may
run alongside an edge no closer than this without being judged to hug.