Remove toPlainString, unify string representation under toString

[Merkost]

Summary

• Preserve trailing zeros — Deci("1.50").toString() now returns "1.50" (removed stripTrailingZeros() from JVM/Android constructors)
• toString() never uses scientific notation on any platform (JVM delegates to BigDecimal.toPlainString(), JS/wasmJs use toFixed())
• Remove toPlainString() — was added in 0.2.0 but never published to Maven, so no breaking change for users. toString() is the single string representation method.
• Migrate all internal call sites, tests, and sample app
• Update API dumps and CHANGELOG

Test plan

• JVM tests pass (761 tests)
• ktlint check passes
• API check passes (JVM + Android dumps updated)
• Full allTests across all targets (JS, wasmJs, native)
• Verify serialization round-trip preserves trailing zeros

[Copilot]

Pull request overview

This PR removes Deci.toPlainString() from the public API and standardizes all string rendering through toString(), with the intent that toString() is plain-decimal (no scientific notation) and preserves trailing zeros.

Changes:

• Removed toPlainString() from the expect API and platform actuals; updated call sites to use toString().
• Updated JVM/Android to preserve trailing zeros by removing stripTrailingZeros() in string constructors and delegating toString() to BigDecimal.toPlainString().
• Updated tests, sample app, API dumps, and documentation/CHANGELOG for the breaking change.

Reviewed changes

Copilot reviewed 22 out of 22 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
sample/composeApp/src/commonMain/kotlin/org/kimplify/screens/ValidationScreen.kt	Updates sample serialization UI to compare via toString() instead of toPlainString().
docs/superpowers/specs/2026-03-27-remove-toplainstring-design.md	Adds design spec describing the API removal and new toString() guarantees.
docs/superpowers/plans/2026-03-27-remove-toplainstring.md	Adds an implementation plan/checklist for the migration and platform changes.
deci/src/wasmJsMain/kotlin/org/kimplify/deci/Deci.kt	Removes toPlainString() and changes toString() to use toFixed() to avoid scientific notation.
deci/src/jvmMain/kotlin/org/kimplify/deci/Deci.kt	Preserves trailing zeros by removing stripTrailingZeros() and makes toString() plain-decimal.
deci/src/jsMain/kotlin/org/kimplify/deci/Deci.kt	Removes toPlainString() and changes toString() to use toFixed() to avoid scientific notation.
deci/src/commonTest/kotlin/org/kimplify/deci/math/DeciMathExtendedTest.kt	Migrates test call site to toString().
deci/src/commonTest/kotlin/org/kimplify/deci/bulk/DeciBulkOperationsTest.kt	Migrates test call site to toString().
deci/src/commonTest/kotlin/org/kimplify/deci/DeciTest.kt	Updates expectations for division scale and trailing-zero preservation; removes toPlainString() usages.
deci/src/commonTest/kotlin/org/kimplify/deci/DeciSerializationTest.kt	Migrates assertion from toPlainString() to toString().
deci/src/commonTest/kotlin/org/kimplify/deci/DeciPropertyTest.kt	Migrates copy-construction to use toString().
deci/src/commonMain/kotlin/org/kimplify/deci/validation/DeciValidation.kt	Migrates validation logic/messages to toString().
deci/src/commonMain/kotlin/org/kimplify/deci/math/DeciMath.kt	Migrates significant-digit rounding to use toString().
deci/src/commonMain/kotlin/org/kimplify/deci/formatting/DeciFormatting.kt	Migrates formatting helpers to use toString().
deci/src/commonMain/kotlin/org/kimplify/deci/extension/DeciExtensions.kt	Migrates utility methods (scale, precision, etc.) to use toString().
deci/src/commonMain/kotlin/org/kimplify/deci/DeciSerializer.kt	Serializes using toString() and updates KDoc accordingly.
deci/src/commonMain/kotlin/org/kimplify/deci/Deci.kt	Removes toPlainString() from expect and updates toString() KDoc to reflect new contract.
deci/src/appleMain/kotlin/org/kimplify/deci/Deci.kt	Removes Apple actual fun toPlainString().
deci/src/androidMain/kotlin/org/kimplify/deci/Deci.kt	Preserves trailing zeros by removing stripTrailingZeros() and makes toString() plain-decimal.
deci/api/jvm/deci.api	Updates API dump to remove toPlainString() from the JVM public API.
deci/api/android/deci.api	Updates API dump to remove toPlainString() from the Android public API.
CHANGELOG.md	Documents breaking changes and migration guidance.

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