Export-NetBox: IPv6 → primary_ip6; don’t map Reachable=$null to offline #72

Context

Post #57/#68 dual-stack work, Export-NetBox still places IPv6 addresses into primary_ip4 (and leaves primary_ip6 unused). Separately, after #66 tri-state reachability, NetBox export maps Reachable=$null to offline — same honesty bug the wizard fixed.

Tip baseline: c47f0a1 (post #65–#71).

What

1. Put IPv6 device IPs in primary_ip6; leave primary_ip4 empty when the node is v6-only. Dual-stack nodes: IPv4 → primary_ip4, IPv6 → primary_ip6 (document if a node can only carry one primary — prefer address family of the node IP).
2. Do not map Reachable=$null to offline; preserve Unknown/indeterminate (omit status or use NetBox’s equivalent of unknown — match existing status vocabulary without inventing new schema).
3. Pester: assert NetBox JSON/columns for v6-only, v4-only, and dual-stack fixtures; assert $null Reachable does not become offline.

Acceptance criteria

• IPv6-only node → primary_ip6 set, primary_ip4 empty/absent
• IPv4-only node unchanged
• Reachable=$null does not export as offline
• Pester covers IPv6 columns + null-reachable regression

Out of scope

Gallery release (separate). SNMP walk / cred-map IPv6.

Related

Owl cycle 2 P0#1 + P1#6 + Pester fold. Follows #57, #66, #68.