feat: add ledger and exchange domain

[ShashankShekhar31]

Summary

• Add ledger and exchange domain foundation.
• Add exchange accounts, subaccounts, asset accounts, deposits, withdrawals, trades, and fees.
• Add immutable ledger domain types and posting validation/builders.
• Add reconciliation contracts, engine, repository, and PostgreSQL persistence.
• Add withdrawal segregation-of-duties constraints.
• Add comprehensive unit and PostgreSQL repository tests.

Validation

• pnpm typecheck ✅
• pnpm test ✅
• Focused exchange/reconciliation tests: 39 tests passed
• git diff --cached --check clean
• Working tree clean before push

Architecture

• PostgreSQL remains authoritative for ledger/accounting state.
• Exchange/custody remains a separate trust domain from the self-custody wallet.
• Reconciliation is defined as an architecture contract; no real custodial exchange infrastructure is introduced.