Reproduce fresh identity setup and SAP outbox replay #31

Tests for #7, stacked on its product branch. The fixture mints fresh Habitat identities and asserts exact record URI/content through the real imported SAP outbox, repeat polling, replay of the same outbox ID, acknowledgement and absence after restart. The focused Go test checks the usable notification handler at the pinned revision. On the dedicated b1 guest: initial observation failed for 90 polls before explicit TrackSpace and DID TLS; after the prerequisites were fixed, rows 3 and 4 arrived. Row 3 survived a process restart and a guest reboot, then remained absent after ack and another restart. The existing 11 CLI tests and one spike Go test passed. docs/SPACE-SPIKE.md contains prerequisites, actual behavior, boot IDs and reproduction; supporting infra tests are notplants-nix #42.