Add custom currency support via custom_currency_path

[csgavino]

Adds support for app-defined custom currencies (e.g. loyalty points) loaded from a user-provided YAML file. Follows the same pattern established in #401 for crypto currencies: config option gates the feature, Loader reads the file, data is frozen via deep_deduplicate!.

Changes

• Added custom_currency_path config option (defaults to nil)
• Added Loader.load_custom_currencies(path) using YAML.safe_load_file for security
• Updated Currency and Currency::Loader to handle custom currencies as a third-tier fallback (ISO > crypto > custom)
• Path-keyed caching (@@custom_currencies_cache[path]) for fiber safety
• Added comprehensive test coverage for new functionality
• Updated RBS type signatures

Breaking changes

None. The feature is opt-in and disabled by default, so existing code will continue to work without any changes.

Configuration System

Before

# No built-in custom currency support
Money.new(500, "CREDITS") # Raises UnknownCurrency

After

# Point to a YAML file with your custom currencies
Money.configure do |config|
  config.custom_currency_path = "/path/to/custom_currencies.yml"
end

Money.new(500, "CREDITS") # Works!

YAML format

credits:
  iso_code: "CREDITS"
  name: "Loyalty Points"
  symbol: "CR"
  disambiguate_symbol: "CR"
  subunit_to_unit: 1
  smallest_denomination: 1
  decimal_mark: "."

Design decisions

• YAML.safe_load_file for user-provided paths (bundled YAML files still use YAML.load_file)
• Custom currencies cannot shadow ISO or crypto currencies — lookup order is ISO > crypto > custom, so a custom "USD" entry is ignored
• ArgumentError raised if the YAML file doesn't contain a Hash
• Path-keyed cache instead of a single class variable, so different fibers with different custom_currency_path values don't share stale data

[kajol-maheshwari]

I think we should add some documentation for configuration and the shape of custom currencies file

[csgavino]

The PR description includes config examples and the YAML format — is there a specific place you'd like documentation added (README, inline comments, etc.)?

[kajol-maheshwari]

@csgavino yes in the README, like we have for crypto currencies https://github.com/Shopify/money?tab=readme-ov-file#crypto-currencies