docs(wiki): subscription recurring billing + party ledger — design only

Subscription page: the one-window model can't express prepaid/postpaid, calendar or
anniversary anchoring, grace or an expiry notice; and renewal is OFF-BOOK today (a PUT
that appends no payment — the same hole closed for the first sale on 2026-06-20).
Designed: plan billing rule, per-day pricing so both anchors share one formula, open-
ended agreement, subscription_periods where each period is a ledger charge and a renewal
= paying the next period, one subscriptionAccess() gate function, expiry notice derived
not stored.

New decision page party-ledger: a counterparty sub-ledger for who-owes-whom across
modules — parties + signed charge / settlement / write_off events, balance derived never
stored, aging + statements + CSV; lands postpaid subscriptions, hotel guest-nights,
fleet washes on account, supplier/utility bills. Sub-ledger only: no bookkeeping, a
statement is not a fiscal invoice, parties per appliance. validation-sponsorship's
sponsor table marked superseded; open-questions #17; index.

Claude-Session: https://claude.ai/code/session_01FWncR69HgGPuei1dLrW3cU

wiki/decisions/open-questions.md

@@ -2,7 +2,7 @@
 type: decision
 tags: [parking, decisions, open]
 sources: [parking-system-architecture]
-updated: 2026-09-04
+updated: 2026-09-08
 status: open
 ---
 
@@ -148,3 +148,11 @@ procurement. (See [[parking-system-architecture]] §10.)
     already carried `subscription:*` (stale note); roles now remember the jobs they follow and
     a grown job is re-applied with one click, never silently; every role edit is signed as a
     `config_change`. **Settled** — details on [[venue-modules]] §"Permissions matrix" Status.
+17. **Party ledger — receivables & payables across modules.** _(Raised by the user, 2026-09-08.)_
+    Postpaid [[subscription]]s, hotel guest-nights billed to the hotel, Car Wash fleet deals on
+    account, and supplier/utility bills all need "who owes whom". Designed as a **counterparty
+    sub-ledger** — parties + signed `charge` / settlement / `write_off` events, balance derived,
+    aging + statements, CSV for the accountant — see [[party-ledger]] (design only, not built).
+    Interacts with #9 (a statement is **not** a fiscal invoice; fiscalisation is off-appliance)
+    and #8 (one currency per party until FX exists). Also reopened on the subscription page: a
+    **renewal is currently off-book** (an edit, no `payment`).