/chapter: plan the next chapter

Nothing here needs the climax in memory: the brief carries what the plan must honour, so this runs as well in a fresh session as at the end of the checkpoint.

1. git pull --rebase origin main, then python3 engine/saga.py now. The previous chapter's climax must be recorded (saga.py record with its choice) and its week frozen (darrow.py chapter-close). If that climax was the Book's last (the KNOT line said the transition was due and route decide --book N ran at the checkpoint): python3 engine/saga.py plan book open N+1 first (it seeds the new Book's beats and quests from the arc and folds the old Book's unwritten beats); the crossing's ### Choice block is written when he answers, or by its default at the new chapter's first close.
2. python3 engine/saga.py brief --chapter: the Book's beats left and the compress rule when a Knot nears (at most two core beats a chapter: one in its week as a beat choice on a spine slot, never Saturday's, one as the climax), the shape rules, the device kinds that rest this chapter, the quests that may run (gates met, required first, never a third live), a Tether stage waiting for a float slot, the open climax choice (it carries two scenes), the threads owed a touch with their plant counts, the arc's next beat, and the template with the device-allowed slots marked. On demand: python3 engine/saga.py arc <id> for a beat or a quest, threads --id N, history --thread N, lore (the index, once per chapter).
3. Fill the template in a scratch file: a one-line question (a sub-question of the Book's); 7 slots Sun–Sat with kind (≥ 2 spine, Saturday always spine, ≤ 1 interlude, two quest slots float: true), each with its beat or quest+stage, a one-line plan from the arc and a one-line turn (who can, must, believes, risks or chooses what by the end that they could not at the start, and what Darrow attempts); micros in non-adjacent slots touching ≥ 2 axes, each with its default; the devices by the pacing rule (one per slot, none on neighbouring days, at most two of challenge / invitation / project, one challenge, two invitations, no kind the last chapter carried): a challenge only on an Art the page has named, an invitation only from someone with a character file who is alive, where Darrow is by the page, with a why_now from their want and the night the spine has made, a project stage only at a place on the page and never in two chapters running; world_moves (three, each a thing the world did); the climax plan with checks, 2–3 options and a default (the option the highest-approval companion present would pick). A slot reveals a truth only when its thread shows two plants on the page; otherwise it plants. A Tether stage only in order and once its stage gate is met. The chapter touches ≥ 1 open thread and plants at most one new one. A slot's plan may name entries of the Annals as lore:<id>.
4. python3 engine/saga.py plan chapter open --number N '<json>' (it refuses a third live quest and an open micro, warns about empty turns, locked gates and a repeated device kind; check refuses the pacing and the gates). Then the chapter file saga/chronicle/NN-<slug>.md: # Chapter N — Title, the epigraph (1–3 italic lines from an in-world document, a — source line), ---; a new Book opens with # BOOK N — TITLE above it. python3 engine/saga.py check, then python3 engine/darrow.py ship -m "chapter N planned: <title>".
5. Afterwards a planned slot is edited with python3 engine/saga.py plan slot N key=value … (--force for a written or skipped one); a Knot tied mid-week re-plans the rest of the week toward the crossing only when the KNOT line says the transition is due at this chapter's climax.