Add CodingTransformer map helpers

[WendellXY]

Summary

• Add CodingTransformer.map(_:) for lightweight output projections.
• Add optional-output shortcuts for default-before-map and failable-map-with-fallback pipelines.
• Cover success, upstream failure, thrown mapping failure, optional defaults, and enum raw-value fallback behavior.
• Document the concise account-type mapping pattern in the README.

Tests

• swift format lint --strict Sources/CodableKit/Transformers/CodingTransformer.swift Sources/CodableKit/Transformers/CodingTransformerComposition.swift Tests/TransformerTests/DerivedValueTransformerTests.swift
• git diff --check
• swift test --filter DerivedValueTransformerTests
• swift test

[Copilot]

Pull request overview

This PR extends CodingTransformer with convenient map helpers to enable lightweight output projections and common optional/default mapping pipelines, and documents the pattern for derived properties.

Changes:

• Add CodingTransformer.map(_:) to map successful outputs while propagating upstream failures and mapping-thrown errors.
• Add map(defaultValue:_:) and map(defaultValue:fallbackValue:_:) to streamline optional-output pipelines (default-before-map and failable-map-with-fallback).
• Add targeted tests for success/failure propagation and optional/fallback behaviors, plus README documentation for the derived-key usage pattern.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
Sources/CodableKit/Transformers/CodingTransformer.swift	Adds the public map helper overloads on CodingTransformer.
Sources/CodableKit/Transformers/CodingTransformerComposition.swift	Introduces MapTransformer implementation used by map(_:) and removes redundant explicit initializers (memberwise inits suffice).
Tests/TransformerTests/DerivedValueTransformerTests.swift	Adds test coverage for mapping success, upstream failure propagation, thrown mapping failures, and optional/fallback variants.
README.md	Documents the new .map helpers with a concise derived-property example.

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