tur/json-string

owned String sibling for the tur/json encoder.

json/encode returns a freshly-allocated cstr the caller must free (a malloc'd growable buffer typed as a borrowed cstr -- the exact hazard the owned String type fixes; see string-adoption-stdlib-plan.md). Serialized output is almost always returned, stored, or written past the call frame, which is where a borrowed-typed owned buffer becomes a leak/lifetime hazard. This opt-in module returns it as an owned String, safe to store and pass on.

Separate module so the lean tur/json stays free of the map/typeclass dependency; only programs wanting an owned encode result load this.

Note: json/encode is inline-C, so like tur/json itself this runs on the compiled path; use the underlying json/encode cstr form under --interpret.

Since: v2 (string-adoption-stdlib-plan, batch 3)