feat(relation): refuse unobserved pairs as no-relationship

[seonghobae]

Summary

ADR 0003 remaining identity gate: observed, inferred, and unobserved relation statuses stay distinct. Unobserved pairs are missing statuses, not negative edges.

• refuse_absence_as_negative fails closed on Unobserved.
• Observed and inferred presence evidence stay typed and are not treated as absence.
• Recovered statuses match known truth at a higher computed rate than collapsing every status to observed.
• No new migration (0008 not allocated). Does not replace relation_graph.

Test plan

• RED: absence_status_contract failed to compile (E0432) before the crate API existed
• cargo test -p relation_absence --offline (lib + 2 contract tests)
• cargo clippy -p relation_absence --all-targets --offline -- -D warnings
• python3 scripts/check_workspace_contract.py / check_docstrings.py / validate_documentation.py
• focused llvm-cov lines 14/14 and nightly branches 6/6
• exact-head required Checks on this PR (queued; do not treat as passed)

Do not self-approve or merge until exact-head required checks and a qualifying independent review pass. Keep #46 ready; do not empty-commit it.

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