feat: expand payment channel coverage to additional markets and currencies

[isaacknjama]

Problem

The registry currently covers three markets — Kenya (KES), Malawi (MWK), and South Africa (ZAR) — each added market-by-market with its own mobile money, bank, and cash channels (src/channels/ke, src/channels/mw, src/channels/za). CurrencyCode and CountryCode in core.ts are typed as open Uppercase<string>, so nothing in the type system or registry stops a market from being added, but every additional currency still requires hand-authoring channel schemas from scratch. Consumers outside these three markets currently have no built-in channels at all.

Goal

Extend the built-in registry so it is not limited to KES, MWK, and ZAR — add payment channel schemas for the remaining currencies/markets the package is meant to serve, following the existing per-country structure and reusable composition helpers (src/channels/shared.ts, e.g. createCashPaymentChannel).

Proposed approach

• Identify the target list of markets/currencies to add (e.g. additional African markets, or a specific set requested by consuming applications — needs confirmation/prioritization).
• For each new market, add a src/channels/<country>/ directory following the existing pattern: one file per network/channel variant, plus a shared cash channel via createCashPaymentChannel.
• Reuse src/channels/shared.ts composition helpers where field shapes overlap (phone fields, bank account fields, masking/normalization kinds) instead of duplicating definitions per country.
• Add any new NormalizationKind values required for market-specific phone/account formats (mirroring E164KenyaPhone / E164MalawiPhone).
• Register each new schema's stable ID using the existing <network>_<variant?>_<country>_<currency> convention.
• Update README.md's market inventory/examples and src/index.ts exports as new channels are added.

Acceptance criteria

• Agreed list of new markets/currencies to add is documented on this issue.
• Each new market has at least one channel schema (mobile money, bank, or card as applicable) plus a cash channel.
• New schemas pass existing validation/normalization test coverage patterns (see test/).
• README.md and any generated registry docs reflect the added currencies.
• No breaking changes to existing KE/MW/ZA channel IDs or schema versions.

Out of scope

• Introducing a canonical ISO 4217 currency enum/constant list (tracked separately if needed).
• Provider integrations, compliance, or settlement logic — this package only defines declarative schemas.