I'm writing a series — how do I plan that a character dies (or marries) in a specific volume?

With arc milestones. Open any book of the series, click "Codex", switch the scope to "Series" at the top, expand the character's card and use "Arc milestones": pick the volume number and describe the event — "Book 3: gets sick and dies", "Book 4: marries Mara". BookyAI then treats the event differently depending on which volume it is writing: books BEFORE that volume may only foreshadow it (it cannot happen early), the anchored volume MUST make it happen, and every book AFTER treats it as a permanent established fact — a dead character stays dead, a marriage holds. The milestones also appear on the Codex Timeline tab with "ahead / this book / done" badges, and the Consistency check flags any chapter that contradicts them.