Previous modifier

[ra1028]

Pull Request Type

• Bug fix
• New feature
• Refactoring
• Documentation update
• Chore

[Copilot]

Pull Request Overview

This PR introduces a new "previous" modifier for atoms that provides access to the previous value of an atom rather than the current value. The modifier returns an optional type since there may not be a previous value initially.

Key changes:

• Implements PreviousModifier that tracks and returns the previous value of any atom
• Adds comprehensive test coverage for the modifier functionality
• Updates documentation to include the new modifier

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
Sources/Atoms/Modifier/PreviousModifier.swift	Core implementation of the previous value modifier with storage management
Tests/AtomsTests/Modifier/PreviousModifierTests.swift	Complete test suite covering basic functionality, multiple watchers, and downstream updates
Sources/Atoms/Atoms.docc/Atoms.md	Documentation index updates to include the new modifier
README.md	User-facing documentation with usage examples

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