* Fix entity_id null constraint for non-ASCII entity names (Turkish İ etc.)
Python's str.lower() and PostgreSQL's LOWER() produce different results for
some Unicode characters. The most common case is Turkish İ (U+0130):
Python: 'İstanbul'.lower() == 'i\u0307stanbul' (i + combining dot, 2 chars)
PostgreSQL: LOWER('İstanbul') == 'istanbul' (plain i, 1 char)
In _resolve_from_candidates, the fallback SELECT for conflicted entity names
passed Python-lowercased strings to LOWER(canonical_name) = ANY($names), so
PostgreSQL couldn't match them. entity_ids[idx] stayed None, which then
caused a NOT NULL violation on unit_entities.entity_id, failing the entire
retain.
Fix: pass original mixed-case names to the fallback SELECT and use
LOWER(canonical_name) = ANY(SELECT LOWER(n) FROM unnest($2) AS n) so
PostgreSQL lowercases both sides identically. The query also returns the
original input_name so we can add a Python-lowercased key to id_by_name
for the assignment loop that uses Python-lowercased keys.
* Add regression test for Unicode entity conflict
