feat(channels): add cash channels for Central American markets

[altradits]

Summary

Toward #29.

Adds built-in cash channels for Central America (UN M49: 013), covering all 8 countries/territories and 9 currency pairs:

1. Belize (BZ / BZD): cash_bz_bzd — Belize Dollar
2. Costa Rica (CR / CRC): cash_cr_crc — Costa Rican Colón
3. El Salvador (SV / USD): cash_sv_usd — United States Dollar
4. Guatemala (GT / GTQ): cash_gt_gtq — Guatemalan Quetzal
5. Honduras (HN / HNL): cash_hn_hnl — Honduran Lempira
6. Mexico (MX / MXN): cash_mx_mxn — Mexican Peso
7. Nicaragua (NI / NIO): cash_ni_nio — Nicaraguan Córdoba
8. Panama (PA / PAB): cash_pa_pab — Panamanian Balboa
9. Panama (PA / USD): cash_pa_usd — United States Dollar

Implementation Details

• Uses the canonical createCashPaymentChannel helper (schema version 2, automation: PaymentChannelAutomation.None, no collection fields).
• Adopts the multi-currency source layout src/channels/<country>/cash_<currency>.ts for Panama (pa/cash_pab.ts and pa/cash_usd.ts) to avoid collision between dual legal tenders. Single-currency markets continue using cash.ts.
• Updates source-layout contract test in test/payment-channels.test.mjs and documents convention in README.md.
• Re-exports all schemas from src/index.ts and registers them in builtinPaymentChannels.
• Adds discovery and regression tests in test/payment-channels.test.mjs for Central American markets and Panama dual cash resolution.
• Refreshes generated registry documentation via npm run docs:generate.

Authoritative Documentation & Primary Sources

• Belize: Central Bank of Belize (Central Bank of Belize Act, Cap. 262).
• Costa Rica: Banco Central de Costa Rica (Ley Orgánica del Banco Central de Costa Rica, Ley 7558).
• El Salvador: Banco Central de Reserva de El Salvador (Ley de Integración Monetaria, Decreto No. 201).
• Guatemala: Banco de Guatemala (Ley Monetaria, Decreto 17-2002).
• Honduras: Banco Central de Honduras (Ley del Banco Central de Honduras, Decreto 53).
• Mexico: Banco de México (Ley Monetaria de los Estados Unidos Mexicanos).
• Nicaragua: Banco Central de Nicaragua (Ley Orgánica del Banco Central de Nicaragua, Ley No. 732).
• Panama: Banco Nacional de Panamá (Constitución Política de la República de Panamá, Art. 262; Código Fiscal, Art. 1171, establishing Balboa and US Dollar as dual circulating legal tender).

Verification

• npm run check (formatter, TypeScript typecheck, docs check, examples check, and 49/49 tests pass)
• npm pack --dry-run (clean tarball with runtime artifacts only)
• npm run verify:pack (verified on isolated ESM and CommonJS consumers)