Summary

• insert_or_supersede_with_metrics violated uq_graph_edges_active_head (partial unique index on (source_entity_id, target_entity_id, canonical_relation, edge_type) WHERE valid_to IS NULL AND expired_at IS NULL) because insert_new_edge ran before the prior head was deactivated
• SQLite enforces unique indexes at statement level, not at transaction commit — a DEFERRED transaction does not help
• Fix: split invalidate_prior_head into expire_prior_head (sets valid_to/expired_at before INSERT) and set_superseded_by (back-fills superseded_by after INSERT), both within the same transaction

Root cause

// Before (buggy):
insert_new_edge(...)          -- unique index violated if old head still active
invalidate_prior_head(...)    -- too late

// After (correct):
expire_prior_head(...)        -- deactivates old head; index now clear
insert_new_edge(...)          -- no conflict
set_superseded_by(...)        -- back-fills superseded_by with new row ID

Test plan

• cargo nextest run -p zeph-memory --lib — 1272 passed, regression test insert_or_supersede_same_tuple_no_unique_index_violation covers the exact bug scenario
• cargo +nightly fmt --check — CLEAN
• cargo clippy -p zeph-memory --tests -- -D warnings — CLEAN

Closes #3635