Use currency-specific decimal separators

[yuefdev]

Fixes #586.

French (Switzerland) defines currencyDecimal = '.', but currency formatting currently uses its ordinary decimal separator, ','. For example, USD 12345.678 renders as 12'345,68 $US instead of 12'345.68 $US.

Use the currency-specific symbol when a currency is supplied to NumberPattern.apply, passing it through fractional formatting. This covers standard, accounting and long-name currency formats, as well as scientific and significant-digit custom patterns. Ordinary decimal formatting keeps the locale's decimal symbol.

Six new regression cases fail before the fix.

Validation:

• python -m pytest tests/test_numbers.py -q --tb=short --show-capture=no: 137 passed.
• Ruff lint and the repository's configured pre-commit hooks passed for both changed files.

AI assistance: Codex prepared the fix and tests and ran the automated checks above. No independent human review is claimed.