The remote tracker increments imm_words/imm_kanji.frequency live, so a
snapshot holding a stale ACTIVE session (skipped by the merge) has that
session's partial occurrences baked into frequency. A word new to the
local DB adopted that full total, then received the same occurrences
again when the session finalized and merged on a later sync.
Subtract active-session occurrence counts when adopting a new row; they
are re-added exactly once when the session completes. Existing rows were
already safe (isNew guard in addWordOccurrences/addKanjiOccurrences).
Verified red/green: the new two-sync regression test fails without the
subtraction (frequency 9 instead of 5).
