SessionManager.fork() and createBranchedSession() deep-copy a parent's
entries into the child JSONL — same entry_id, timestamp, model,
responseId, token counts, cost. Stats keyed uniqueness on
(session_file, entry_id), so both files contributed to request, token,
and cost totals.
insertMessageStats and insertUserMessageStats now skip rows whose
(entry_id, timestamp) already exists under a different session_file
(first-write-wins). A one-shot migration on initDb, gated by the
fork_dedupe_v1 meta sentinel, collapses pre-existing duplicate rows
in messages and user_messages by keeping the lowest-id row per
(entry_id, timestamp) group.
Fixes#3370
roboomp
b5769a4e10