Add stateful property-based tests using rapid

Introduces two property-based tests using rapid's state machine support:

TestLedgerStateMachine generates random sequences of actions (create
account, create transfer, batch transfers, constrained accounts,
expected-failure transfers) and checks five invariants after every step:
- Entry chain consistency (prev + amount = curr for each entry)
- Zero-sum balance (sum of all balances per currency is zero)
- Version tracking (account version matches entry count)
- Two entries per transfer (debit + credit summing to zero)
- Entry version monotonicity (strictly increasing, no gaps)

TestLedgerConcurrentStateMachine runs parallel goroutines performing
transfers against shared accounts, then verifies the same invariants
hold — stress-testing the sorted-ID locking strategy.

Author: pgr0ss

.github/workflows/run-tests.yml

@@ -45,6 +45,13 @@ jobs:
         env:
           POSTGRES_VERSION: ${{ matrix.postgres_version }}
 
+      - name: Upload rapid failure reproductions
+        if: failure()
+        uses: actions/upload-artifact@v6
+        with:
+          name: rapid-testdata-pg${{ matrix.postgres_version }}
+          path: go/test/testdata/rapid/
+
   all-tests-passed:
     if: ${{ always() }}
     needs: test

.gitignore

@@ -1 +1,2 @@
-.aider*
+/.claude/settings.local.json
+/go/test/testdata/rapid/

go/go.mod

@@ -5,6 +5,7 @@ go 1.26.1
 require (
 	github.com/jackc/pgx/v5 v5.9.1
 	github.com/stretchr/testify v1.11.1
+	pgregory.net/rapid v1.2.0
 )
 
 require (

go/go.sum

@@ -43,3 +43,5 @@ gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c/go.mod h1:JHkPIbrfpd72SG/EV
 gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
 gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
+pgregory.net/rapid v1.2.0 h1:keKAYRcjm+e1F0oAuU5F5+YPAWcyxNNRK2wud503Gnk=
+pgregory.net/rapid v1.2.0/go.mod h1:PY5XlDGj0+V1FCq0o192FdRhpKHGTRIWBgqjDBTrq04=