docs(recurring): record the invariants and what they cost

Two rules that only review catches, so they go in AGENTS.md §1: occurrences are
computed and never stored, and a confirmation's date and its slot are different
facts. Collapsing the second pair into one field would silently delete the
phone-bill behaviour, which is why it is written down rather than left to the
types.

§2 gets the termination guard. n >= 1 is rejected at both write boundaries
rather than clamped, and the walk throws rather than breaking, because a bill
silently dropped from a budget is worse than a visible error.

Demo data seeds a monthly rent and a 28-day subscription — the second is what
puts two occurrences in one month, which is the case worth looking at. It also
seeds a biweekly cost and a lastCharge-anchored one, so every recurrence kind
and both anchorings show up in Settings and the month view.

DECISIONS.md, PRODUCT.md, ARCHITECTURE.md and TODO.md are updated to match:
schema is at 7, the month view carries a second Projected balance beside
Remaining, and TODO.md records what a week of real use — not a test — will
have to answer.

docs/PRODUCT.md

@@ -67,10 +67,18 @@ Confirmed and in place:
   browser.
 - Offline-capable installable PWA. Currently deploys as one static artifact to
   both GitHub Pages (under a subpath) and a self-hosted Docker image.
+- Recurring costs — a bill that repeats monthly, every N days, or every N
+  weeks on a weekday, split across posts like any purchase. Each due date is a
+  projected occurrence the owner confirms into a real purchase; the month view
+  shows what is still expected alongside a second, projected balance per post,
+  so a known bill is visible before it is paid without touching the reconciled
+  figure a bank statement is checked against.
 
 Terminology: an envelope is called a **post**. A **finance plan** is a purchase
 spread across months. An **override** is a one-month exception to a post's
-allocation.
+allocation. A **recurring cost** is a bill expected to repeat; each due date it
+projects is an **occurrence**; **confirming** an occurrence turns it into a
+real purchase.
 
 Undecided: nothing outstanding at the product level.