D-111: align VR1 v6 subcarve to deployed NN mnemonic; fix ULA-gen doc bug ...

Basing VR1 NetBox on the real netbox.baldurkeep.com modeling (operator
directive). A read-only live export confirmed VR0-DC0/Willamette use an NN
net-byte mnemonic with /60-per-plane + /64-active; dc-dc-prefixes-import.py
did not match it (arbitrary contiguous indices, /64-only).

DOCFIX-182 / D-111: carve_v6 replaces carve_ula/carve_gua -- provider :10
(+API-VIP :11) GUA, metal /60 shared by admin :20 / internal :21, data :30,
storage :40, repl :50, all /60+/64 ULA; the ULA /56 is indexed to the GUA site
nibble so the 4th hextet reads DC.NN in both families. Direct-math _sub_at (no
subnet enumeration -- preserves the DOCFIX-181 no-hang property). Harness
40 -> 53 PASS. D-111 ADOPTED in design-decisions.md; scope-doc sub-decision #1
ratified.

DOCFIX-183: fix the invalid-IPv6 ULA-generation sed in the netem/ULA proposal
doc (4+6 -> 2+4+4 hextets, caught by actually running it); ratified VR1 ULA is
fd50:840e:74e2::/48.

repo-lint 0 fail; tests/dc-dc-prefixes-import 53/53.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>