Draft: Close #955 - Map key constraint

[cchantep]

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Fixes

Fixes #955

Purpose

Fix implicit resolution of KeyReads / KeyWrites for types represented as JSON strings.

readableKeyReads currently derives a KeyReads[T] from any Reads[T]. This is too broad: types such as tuples have a Reads, but are not represented as JSON strings and therefore cannot be used as JSON object keys through this mechanism.

This PR introduces Format.Representation[T, R] as explicit evidence that a type has a given JSON representation, and uses it to constrain the string-based KeyReads / KeyWrites derivation.

The existing readableKeyReads is kept for backward compatibility but deprecated.

Background Context

A Reads[T] does not imply that T can be read from a JsString. For example, Reads[(Int, Int)] exists even though (Int, Int) is not a string representation (unlike for example URI which is represented as JSON string).

The new Representation[T, JsString] makes this distinction explicit and prevents KeyReads / KeyWrites from being derived for types that are not string-represented.

Representations for types whose JSON representation depends on the Writes / Reads (e.g. temporal types, duration, ...) in scope are provided separately through Format.Representation.Implicits, so they are opt-in rather than unconditional.