[Refactor/#1] PickerState to index-based generic state with reliable onValueChange

[DongChyeon]

🎯 Related Issue

• Closes ♻️ Refactor - PickerState to use index-based state #1

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📝 Description

What does this PR do?

Refactor PickerState to use index-based unidirectional state management for better consistency with Compose paradigms.
Make PickerState generic for type safety and reusability.
Refactor PickerItem scroll observer logic to calculate adjustedIndex before distinctUntilChanged, preventing skipped onValueChange triggers.
Ensure getItemForIndex returns an empty string when item is null to avoid displaying "null".

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🔄 Comparison

Before	After
PickerState stored mutable selectedItem directly	Uses selectedIndex: StateFlow<Int> with derived selectedItem getter, enforcing unidirectional data flow
Only supported String items	Now supports generic types with type-safe selection
distinctUntilChanged applied to centerIndex, mapping afterwards	Calculates adjustedIndex before distinctUntilChanged, preventing skipped updates
getItemForIndex used toString() directly, risking "null" display	Returns empty string "" safely when item is null

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✅ Changes

• Refactor PickerState to index-based unidirectional state
• Make PickerState generic for type safety
• Calculate adjustedIndex before distinctUntilChanged in PickerItem
• Return empty string when item is null in getItemForIndex

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🔍 Screenshots / Test Results (if applicable)

N/A

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👤 Reviewer Checklist

N/A

[Copilot]

Pull Request Overview

Refactors the picker components to use an index-based, generic state model and fixes skipped value-change callbacks by calculating adjusted indices before deduplication.

• Converts PickerState to a generic, index-driven design with StateFlow<Int>
• Updates PickerItem to compute adjustedIndex before distinctUntilChanged and safely formats items
• Adapts TimePicker to supply item lists as typed List<T> and use itemFormatter for padding

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
timepicker/src/main/java/com/dongchyeon/timepicker/ui/PickerItem.kt	Made PickerItem generic, refactored scroll observer and onValueChange logic, added null-safe item formatting
timepicker/src/main/java/com/dongchyeon/timepicker/model/PickerState.kt	Changed PickerState to generic type T, replaced mutable item with index-based StateFlow
timepicker/src/main/java/com/dongchyeon/timepicker/TimePicker.kt	Switched hour/minute lists to typed lists, updated rememberPickerState calls, added itemFormatter

Comments suppressed due to low confidence (1)

timepicker/src/main/java/com/dongchyeon/timepicker/model/PickerState.kt:10

• With the new generic and index-based implementation, consider adding unit tests for PickerState<T> and getItemForIndex to verify correct index-to-item mapping, boundary handling, and onValueChange behavior.

class PickerState<T>(