Two Phase-2 followups in one commit since they're coupled at the
write path:
* Agent populates valid_from from the source photo's date_taken
when calling store_fact. Loose semantics — date_taken is *evidence
at that date*, not strictly when the fact started being true — but
gives the curator a calendar anchor and pairs with supersession to
close intervals cleanly. valid_until stays NULL (a single photo
can't tell us when something stopped). Honours the existing
upsert_fact dedup (corroborated facts keep their first-recorded
valid_from).
* Supersession: new column entity_facts.superseded_by INTEGER
(migration 2026-05-10-000200), new status value 'superseded',
new DAO method supersede_fact, new HTTP endpoint
POST /knowledge/facts/{id}/supersede.
Marking an old fact as replaced by a new one atomically: flips
status to 'superseded', sets superseded_by, and stamps
valid_until from the new fact's valid_from (when not already
set). delete_fact clears dangling supersession pointers in the
same transaction so the column never points at a missing row —
no FK because SQLite can't ALTER ADD with REFERENCES, but the
DAO maintains the invariant.
Pairs with conflict detection from the previous slice: once the
old fact's valid_until is closed, its interval no longer overlaps
the new fact's, so they stop flagging — the supersede action
resolves the conflict.
Two tests pin the contract: supersede stamps valid_until from
new.valid_from while respecting an existing valid_until, and
deleting the supersedeR clears the dangling pointer while leaving
the old fact's 'superseded' status in place for history.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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