test(invoice): implement unit tests for invoice behaviors

Add comprehensive unit tests for pure invoice business logic:
- assert-invoice-amounts-add-up (behaviors 9.4, 11.4)
- does-amount-exceed-outstanding? (behavior 13.4)
- assert-percentages-add-up (behavior 15.3)
- stack-rank and deduplicate (behaviors 24.1, 24.4, 24.5)
- clientize-vendor (behavior 8.4)
- location-select* (behavior 9.3)
- maybe-code-accounts with Shared location spreading (behavior 15.6)
- can-undo-autopayment (behaviors 19.2-19.4)
- due date / scheduled payment calculations (behaviors 8.2, 8.3)
- can-handwrite? and credit-only? (pay wizard behaviors)
- due date display logic (behavior 1.7)

Also fixes:
- user.clj: add missing datomic.api alias (d) used in sample functions
- new_invoice_wizard_test.clj: fix sut8 -> sut9 typo

Marks completed unit-test behaviors with [x] in invoice.md

opencode.json

@@ -108,7 +108,11 @@
       "url": "https://mcp.context7.com/mcp",
       "enabled": true
     },
-  
+    "clojure-mcp": {
+      "type": "local",
+      "command": ["clojure", "-Tmcp", "start", ":config-profile", ":cli-assist"],
+      "enabled": true
+    }
   },
   "permission": {
     "read": "allow",