fix(import): preserve orphaned relation audits

[dnlrsls]

🔗 Linked Issue

Closes #1291

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🏷️ PR Type

• type:bug — Bug fix
• type:feature — New feature
• type:question — Question requiring tracked work
• type:docs — Documentation only
• type:refactor — Code refactoring (no behavior change)
• type:chore — Maintenance, dependencies, tooling
• type:breaking-change — Breaking change

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📝 Summary

• Preserve explicitly orphaned relation audit rows during import.
• Keep live graph integrity fail-loud when a non-orphan endpoint is missing.

📂 Changes

File	Change
DOCS.md	Updated by the orphaned relation audit slice.
internal/store/store.go	Updated by the orphaned relation audit slice.
internal/store/store_test.go	Updated by the orphaned relation audit slice.

🧪 Test Plan

• Focused and full store tests pass locally.
• Candidate lint reports 0 issues.
• git diff --check passes.
• User-facing behavior documentation reviewed.

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✅ Contributor Checklist

• Linked approved issue bug(store): engram export --all emits relations with absent endpoints, and import rolls back the entire restore #1291
• Exactly one type:* label
• Commits follow Conventional Commits
• No Co-Authored-By trailers
• Changed paths comply with the Transient Artifact Policy

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Chain Context

Field	Value
Chain	Engram 2.1.0 blocker integration
Position	4 of 13
Base	fix/2.1.0-03-doctor-authority @ c40e12ed
Depends on	#1310
Follow-up	fix/2.1.0-05-cloud-materialization
Review budget	193 / 400 changed lines
Starts at	Doctor authority slice
Ends with	Orphaned relation audit preservation

Scope

• Includes: bug(store): engram export --all emits relations with absent endpoints, and import rolls back the entire restore #1291 only.
• Excludes: later issue slices, merge, tags, release, and publication.

Autonomy

• CI is expected to pass for this PR branch
• One deliverable scope
• Rollback excludes unrelated changes
• Tests cover this unit

💬 Notes for Reviewers

Draft only. Native RDD lineage review-9f5f335b8afd1821 approved without advisories and was acknowledged. Independent verification noted one stale internal store.go comment as a non-blocking readability follow-up; runtime behavior and user documentation are correct. Merge authorization is not granted.

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