feat: support unassigned purchases and allocation status

Allow month-only purchases to start finance plans. Keep unbudgeted posts distinct from explicit zero rules in overspend reporting.

docs/ARCHITECTURE.md

@@ -47,8 +47,9 @@ remaining = carriedIn + allocation − charges     (carried forward, monthly)
 
 Unspent carries; overspend carries as debt. **Nothing else may recompute this.**
 The month view, year matrix, summary, per-post views and the ODS export all
-aggregate over `foldBalances` / `figuresFor`. A closing balance is read from
-`byMonth[11].remaining`, never re-summed.
+aggregate envelope figures over `foldBalances` / `figuresFor`. Overall Spent
+also includes unassigned purchase slices, which have no envelope to fold. A
+closing balance is read from `byMonth[11].remaining`, never re-summed.
 
 A recurring cost adds a SECOND balance beside it, on its own carry:
 
@@ -156,6 +157,17 @@ those parts*. Both split modes collapse to one code path per slice.
 Applying a split's raw value per-slice looks simpler and passes every percentage
 test — and charges a fixed "2000 to Video Games" in full on every slice.
 
+A purchase with no post has an empty `splits` array. Its dated amount counts in
+monthly, yearly and summary Spent totals, but charges no envelope and changes no
+rollover balance. Recurring bills still require a destination post. JSON import
+and ODS export preserve these unassigned purchases.
+
+A post with no applicable allocation rule has no overspend status, even if its
+balance is negative. The balance remains visible and carries normally. An
+explicit zero rule, including a zero monthly override, does have an overspend
+status when its balance is negative. The views use the presence of a rule rather
+than infer it from the numeric allocation, since both cases fold to zero.
+
 ## Things the app deliberately does NOT validate
 
 Adding a "missing" guard here is a regression, not a fix: