-- Add a supersession pointer to entity_facts.
--
-- Status alone is a one-way trapdoor: 'rejected' loses the link
-- between the rejected fact and the one that replaced it. For
-- evolving facts (Cameron's relationship, employer, address) the
-- curator wants to *replace* a stale fact with a new one and keep
-- the history readable: "from 2018 until 2022 this was true, then
-- it became this other thing".
--
-- A nullable INTEGER column pointing at another entity_facts.id —
-- no FK constraint because SQLite can't ALTER ADD COLUMN with REFs;
-- the DAO's delete_fact clears dangling pointers in the same
-- transaction as the parent delete to keep the column honest.
--
-- A status of 'superseded' on the old fact (alongside the existing
-- active / reviewed / rejected) signals "replaced by a newer
-- claim". Read paths already filter 'rejected' out of the active
-- view; the curation UI will treat 'superseded' the same way for
-- conflict detection so they don't keep flagging.
--
-- Pairs with the valid-time columns from 2026-05-10-000100: the
-- supersede action auto-stamps the old fact's `valid_until` from
-- the new fact's `valid_from`, closing the interval cleanly.

ALTER TABLE entity_facts ADD COLUMN superseded_by INTEGER;

-- Helpful index for "show me what superseded this fact" walks
-- (rare today; cheap to add now while the table is small).
CREATE INDEX idx_entity_facts_superseded_by
    ON entity_facts(superseded_by)
    WHERE superseded_by IS NOT NULL;