feat(core): hideZeroFractionDigits option — hide decimals when zero

[Merkost]

Summary

Fixes #5. Adds an opt-in CurrencyFormatOptions.hideZeroFractionDigits flag: when the fractional part is all zeros it's dropped, while non-zero fractions keep full precision.

Input	Currency	Output
34.00	USD	$34
34.20	USD	$34.20
34.88	USD	$34.88

val opts = CurrencyFormatOptions { hideZeroFractionDigits = true }
Kurrency.USD.formatAmountWithOptions("34.00", opts) // "$34"

Design

• The flag is consumed in the common-code CurrencyFormatter.formatWithOptions engine: after the number is rounded and split, an all-zero fractional part is dropped (and so is the decimal separator).
• Done after rounding (reflects displayed digits: 34.004 → $34) and after min/max resolution (so it wins over an explicit minFractionDigits).
• No-op for zero-fraction currencies (JPY); grouping and negatives preserved.
• The platform simple-path (formatAmount/CurrencyStyle) is intentionally not changed — it routes through the platform formatters, and adding the flag there would split one method across two engines.

Why not minFractionDigits?

Setting min=0 would turn 34.20 into 34.2 (strips the trailing zero). This flag is all-or-nothing: drop all fraction digits or keep the currency's full count.

kurrency-deci

Adds CurrencyFormatter.formatWithOptions(amount: Deci, …) overloads (currency code + Kurrency), closing the prior gap where Deci users had no options access — and giving an exact, decimal-native input.

Testing

Extensive commonTest coverage (runs on JVM/JS/WasmJs/iOS): all-zero dropped, non-zero kept, partial trailing zero, grouping preserved, round-down-to-zero, round-up-to-non-zero, zero amount, negatives, 3-digit currency (BHD), comma-decimal locale (German), JPY no-op, hide-wins-over-min, the high-level Kurrency.formatAmountWithOptions entry, serialization round-trip + absent-field default, and Deci (code + Kurrency + high-precision). All green on every platform.

Backward-compatible: new @Serializable field defaults to false.

[Copilot]

Pull request overview

This PR introduces an opt-in CurrencyFormatOptions.hideZeroFractionDigits flag to suppress the decimal separator and fractional digits when the fractional portion is all zeros, while leaving non-zero fractions unchanged. It updates the common formatting engine to apply the behavior after precision resolution and rounding, adds Deci overloads for options-based formatting, and expands cross-platform test coverage including serialization.

Changes:

• Add hideZeroFractionDigits to CurrencyFormatOptions (serializable, default false) and apply it during numeric reassembly in CurrencyFormatter.
• Add CurrencyFormatter.formatWithOptions(amount: Deci, …) overloads in kurrency-deci.
• Add common tests for hide/keep behavior, locale differences, rounding behavior, and serialization defaults/round-trip.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
kurrency-core/src/commonMain/kotlin/org/kimplify/kurrency/CurrencyFormatOptions.kt	Adds hideZeroFractionDigits option to the serializable options model and builder.
kurrency-core/src/commonMain/kotlin/org/kimplify/kurrency/CurrencyFormatter.kt	Drops the fractional part + decimal separator when the fraction is all zeros and the option is enabled.
kurrency-core/src/commonTest/kotlin/org/kimplify/kurrency/HideZeroFractionDigitsTest.kt	Adds cross-locale/common tests covering hide/keep logic, rounding, negatives, and 0-fraction currencies.
kurrency-core/src/commonTest/kotlin/org/kimplify/kurrency/serialization/CurrencyFormatOptionsSerializerTest.kt	Adds serialization round-trip and defaulting coverage for the new option.
kurrency-deci/src/commonMain/kotlin/org/kimplify/kurrency/deci/DeciExtensions.kt	Adds Deci overloads for options-based formatting via CurrencyFormatter.
kurrency-deci/src/commonTest/kotlin/org/kimplify/kurrency/deci/DeciHideZeroFractionDigitsTest.kt	Adds Deci-specific tests validating hide/keep and rounding-to-zero behavior.

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