CI smoke: Import-Inventory dual-stack DrawIO path #75

Context

#67 added IPv6 DrawIO smoke, but it still bypasses Import-Inventory — so #68 dual-stack import is not exercised in the packaged smoke gate.

Tip: c47f0a1.

What

• Smoke path: write dual-stack inventory JSON → Import-Inventory → Export-DrawIO.
• Assert both family IPs appear in XML / no throw.
• Keep offline/no live network.

Acceptance criteria

• Smoke uses Import-Inventory (not a hand-built topology that skips import)
• Dual-stack fixture covers v4 + v6 node/subnet
• Matrix OS set still green offline

Related

Owl cycle 2 P1#4. Follows #62/#67, #68.