chore: enforce final mapper and expand docs

[magicsunday]

M# Sweep — Verify compliance for this milestone

• Pending compliance review

Summary

• mark MagicSunday\\JsonMapper\\JsonMapper and the camelCase name converter as final, switching constructor dependencies to readonly properties for consistent visibility
• expand the README with a JSON→object→collection quick start, converter/class map guidance, error strategies, performance hints, and references to new docs
• add an API reference plus recipe guides for enums, attributes, nested collections, and custom name converters, and record the changes in the changelog

Testing

• composer ci:test:php:unit
• composer ci:test:php:phpstan

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Codex Task